Next.js Image Optimization

Q: What formats does next/image automatically serve, and how does it decide?

It serves WebP or AVIF based on the browser's Accept header. The built-in Image Optimization API handles format conversion automatically. No configuration is needed; it works out of the box.

Q: When should you use the priority prop on an image?

Use priority only on above-the-fold LCP (Largest Contentful Paint) images. Typically one or two images per page. It disables lazy loading and preloads the image. Overusing it defeats the lazy loading benefits.

Q: What CSS requirement does a parent element need for fill mode images?

The parent must have position: relative (or absolute or fixed). It must also have defined dimensions (width/height or aspect ratio). Without this, the image collapses to zero height.

Q: How do static imports differ from remote images in next/image?

Static imports automatically provide width, height, and blurDataURL. Remote images require explicit width and height props (or fill) since Next.js cannot inspect them at build time. placeholder="blur" works automatically with static imports but needs a manual blurDataURL for remote images.

Q: What TypeScript types does next/image export for typing image props?

StaticImageData is the return type of static imports, with src, width, height, and blurDataURL. ImageLoader types the custom loader function.

Recipe

Use the next/image component to automatically optimize images with lazy loading, responsive sizing, and modern formats (WebP/AVIF). Configure remote patterns for external image sources.

// app/components/optimized-image.tsx
import Image from "next/image";
 
// Local image (static import enables automatic blur placeholder)
import heroPhoto from "@/public/images/hero.jpg";
 
export function HeroImage() {
  return (
    <Image
      src={heroPhoto}
      alt="A scenic mountain landscape"
      placeholder="blur"
      priority
      className="rounded-lg"
    />
  );
}

// Remote image with explicit dimensions
export function RemoteImage() {
  return (
    <Image
      src="https://images.unsplash.com/photo-example"
      alt="Unsplash photo"
      width={800}
      height={600}
      className="rounded-lg"
    />
  );
}

// next.config.ts
import type { NextConfig } from "next";
 
const nextConfig: NextConfig = {
  images: {
    remotePatterns: [
      {
        protocol: "https",
        hostname: "images.unsplash.com",
      },
      {
        protocol: "https",
        hostname: "cdn.pixabay.com",
      },
    ],
  },
};
 
export default nextConfig;

Working Example

A responsive image gallery with blur placeholders and a lightbox-ready layout:

// app/components/image-gallery.tsx
import Image from "next/image";
 
interface GalleryImage {
  src: string;
  alt: string;
  blurDataURL: string;
  width: number;
  height: number;
}
 
const images: GalleryImage[] = [
  {
    src: "https://images.unsplash.com/photo-1506744038136-46273834b3fb",
    alt: "Mountain lake at sunrise",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
  {
    src: "https://images.unsplash.com/photo-1469474968028-56623f02e42e",
    alt: "Forest path in autumn",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
  {
    src: "https://images.unsplash.com/photo-1447752875215-b2761acb3c5d",
    alt: "Misty mountain range",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
];
 
export function ImageGallery() {
  return (
    <div className="grid grid-cols-1 gap-4 sm:grid-cols-2 lg:grid-cols-3">
      {images.map((image, index) => (
        <div key={image.src} className="relative aspect-[3/2] overflow-hidden rounded-xl">
          <Image
            src={image.src}
            alt={image.alt}
            fill
            sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw"
            placeholder="blur"
            blurDataURL={image.blurDataURL}
            priority={index === 0}
            className="object-cover transition-transform duration-300 hover:scale-105"
          />
        </div>
      ))}
    </div>
  );
}

// app/gallery/page.tsx
import { ImageGallery } from "@/app/components/image-gallery";
 
export default function GalleryPage() {
  return (
    <main className="mx-auto max-w-6xl px-4 py-8">
      <h1 className="mb-6 text-3xl font-bold">Photo Gallery</h1>
      <ImageGallery />
    </main>
  );
}

Deep Dive

How It Works

next/image automatically serves images in modern formats (WebP, AVIF) based on browser support via the built-in Image Optimization API.
Images are lazy-loaded by default. Only images visible in the viewport (or close to it) are fetched.
The priority prop disables lazy loading and preloads the image, making it ideal for above-the-fold LCP (Largest Contentful Paint) images.
fill mode makes the image fill its parent container. The parent must have position: relative (or absolute or fixed) and defined dimensions.
The sizes prop tells the browser which image width to request at each viewport breakpoint. Without it, Next.js serves the full-size image to all devices.
Static imports (e.g., import hero from "@/public/hero.jpg") automatically provide width, height, and blurDataURL.
Remote images require width and height props (or fill) because Next.js cannot inspect them at build time.

Variations

Fill mode with aspect ratio container:

<div className="relative aspect-video w-full">
  <Image
    src="/images/banner.jpg"
    alt="Banner"
    fill
    sizes="100vw"
    className="object-cover"
  />
</div>

Responsive sizes for different layouts:

// Full-width hero
<Image src={src} alt={alt} fill sizes="100vw" />
 
// Two-column grid
<Image src={src} alt={alt} fill sizes="(max-width: 768px) 100vw, 50vw" />
 
// Three-column with sidebar
<Image src={src} alt={alt} fill sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw" />

Custom loader for external CDN:

import Image from "next/image";
 
const cloudinaryLoader = ({ src, width, quality }: { src: string; width: number; quality?: number }) => {
  return `https://res.cloudinary.com/demo/image/upload/w_${width},q_${quality || 75}/${src}`;
};
 
export function CloudinaryImage() {
  return (
    <Image
      loader={cloudinaryLoader}
      src="sample.jpg"
      alt="Cloudinary image"
      width={800}
      height={600}
    />
  );
}

TypeScript Notes

next/image exports ImageProps and StaticImageData types.
Static imports return StaticImageData with src, width, height, and blurDataURL properties.
The loader function is typed as ImageLoader from next/image.

import type { ImageProps, StaticImageData } from "next/image";
 
interface HeroProps {
  image: StaticImageData;
  alt: string;
  priority?: boolean;
}

Gotchas

Using fill without sizes forces the browser to download the largest image variant. Always pair fill with an appropriate sizes prop.
The parent of a fill image must have position: relative and defined dimensions. Without this, the image collapses to zero height.
placeholder="blur" with remote images requires a blurDataURL prop. Only static imports generate this automatically.
The priority prop should only be used on above-the-fold images (typically one or two per page). Overusing it defeats lazy loading benefits.
remotePatterns in next.config requires a server restart to take effect. Changes are not picked up by hot reload.
Setting width and height does not crop or resize the image element in the DOM. These values define the aspect ratio and request size. Use CSS for visual sizing.
Images from domains not listed in remotePatterns will fail with a 400 error at runtime, not at build time.

Alternatives

Approach	Pros	Cons
next/image	Built-in optimization, lazy loading, format conversion	Complex props, server-side dependency
Native img tag	Simple, no configuration	No optimization, no lazy loading by default
Cloudinary or Imgix	Advanced transformations, CDN delivery	External service, additional cost
@unpic/react	Framework-agnostic, works with multiple CDNs	No built-in optimization server

FAQs

What formats does `next/image` automatically serve, and how does it decide?

It serves WebP or AVIF based on the browser's Accept header.
The built-in Image Optimization API handles format conversion automatically.
No configuration is needed; it works out of the box.

When should you use the `priority` prop on an image?

Use priority only on above-the-fold LCP (Largest Contentful Paint) images.
Typically one or two images per page.
It disables lazy loading and preloads the image.
Overusing it defeats the lazy loading benefits.

Gotcha: What happens if you use `fill` without a `sizes` prop?

The browser downloads the largest image variant for all devices.
Always pair fill with an appropriate sizes prop to serve correctly sized images per breakpoint.

What CSS requirement does a parent element need for `fill` mode images?

The parent must have position: relative (or absolute or fixed).
It must also have defined dimensions (width/height or aspect ratio).
Without this, the image collapses to zero height.

How do static imports differ from remote images in `next/image`?

Static imports automatically provide width, height, and blurDataURL.
Remote images require explicit width and height props (or fill) since Next.js cannot inspect them at build time.
placeholder="blur" works automatically with static imports but needs a manual blurDataURL for remote images.

How do you configure `next/image` to allow images from an external domain?

// next.config.ts
const nextConfig: NextConfig = {
  images: {
    remotePatterns: [
      { protocol: "https", hostname: "images.unsplash.com" },
    ],
  },
};

A server restart is required after changes.

Gotcha: Do `width` and `height` props crop or resize the image in the DOM?

No. They define the aspect ratio and the requested image size from the optimization server.
Use CSS classes for visual sizing in the browser.

How do you use a custom loader for an external CDN like Cloudinary?

const cloudinaryLoader = ({ src, width, quality }: { src: string; width: number; quality?: number }) => {
  return `https://res.cloudinary.com/demo/image/upload/w_${width},q_${quality || 75}/${src}`;
};
 
<Image loader={cloudinaryLoader} src="sample.jpg" alt="Photo" width={800} height={600} />

What TypeScript types does `next/image` export for typing image props?

import type { ImageProps, StaticImageData } from "next/image";

StaticImageData is the return type of static imports, with src, width, height, and blurDataURL.
ImageLoader types the custom loader function.

What is the correct `sizes` value for a three-column grid with a sidebar?

sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw"

This tells the browser to request full-width images on mobile, half on tablet, and one-third on desktop.

What error do you get for images from domains not in `remotePatterns`?

A 400 error at runtime, not at build time.
The image simply fails to load when the page is visited.
Add the domain to remotePatterns in next.config.ts and restart the server.

Recipe

Use the next/image component to automatically optimize images with lazy loading, responsive sizing, and modern formats (WebP/AVIF). Configure remote patterns for external image sources.

// app/components/optimized-image.tsx
import Image from "next/image";
 
// Local image (static import enables automatic blur placeholder)
import heroPhoto from "@/public/images/hero.jpg";
 
export function HeroImage() {
  return (
    <Image
      src={heroPhoto}
      alt="A scenic mountain landscape"
      placeholder="blur"
      priority
      className="rounded-lg"
    />
  );
}

// Remote image with explicit dimensions
export function RemoteImage() {
  return (
    <Image
      src="https://images.unsplash.com/photo-example"
      alt="Unsplash photo"
      width={800}
      height={600}
      className="rounded-lg"
    />
  );
}

// next.config.ts
import type { NextConfig } from "next";
 
const nextConfig: NextConfig = {
  images: {
    remotePatterns: [
      {
        protocol: "https",
        hostname: "images.unsplash.com",
      },
      {
        protocol: "https",
        hostname: "cdn.pixabay.com",
      },
    ],
  },
};
 
export default nextConfig;

Working Example

A responsive image gallery with blur placeholders and a lightbox-ready layout:

// app/components/image-gallery.tsx
import Image from "next/image";
 
interface GalleryImage {
  src: string;
  alt: string;
  blurDataURL: string;
  width: number;
  height: number;
}
 
const images: GalleryImage[] = [
  {
    src: "https://images.unsplash.com/photo-1506744038136-46273834b3fb",
    alt: "Mountain lake at sunrise",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
  {
    src: "https://images.unsplash.com/photo-1469474968028-56623f02e42e",
    alt: "Forest path in autumn",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
  {
    src: "https://images.unsplash.com/photo-1447752875215-b2761acb3c5d",
    alt: "Misty mountain range",
    blurDataURL: "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAMCA...",
    width: 1200,
    height: 800,
  },
];
 
export function ImageGallery() {
  return (
    <div className="grid grid-cols-1 gap-4 sm:grid-cols-2 lg:grid-cols-3">
      {images.map((image, index) => (
        <div key={image.src} className="relative aspect-[3/2] overflow-hidden rounded-xl">
          <Image
            src={image.src}
            alt={image.alt}
            fill
            sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw"
            placeholder="blur"
            blurDataURL={image.blurDataURL}
            priority={index === 0}
            className="object-cover transition-transform duration-300 hover:scale-105"
          />
        </div>
      ))}
    </div>
  );
}

// app/gallery/page.tsx
import { ImageGallery } from "@/app/components/image-gallery";
 
export default function GalleryPage() {
  return (
    <main className="mx-auto max-w-6xl px-4 py-8">
      <h1 className="mb-6 text-3xl font-bold">Photo Gallery</h1>
      <ImageGallery />
    </main>
  );
}

Deep Dive

How It Works

next/image automatically serves images in modern formats (WebP, AVIF) based on browser support via the built-in Image Optimization API.
Images are lazy-loaded by default. Only images visible in the viewport (or close to it) are fetched.
The priority prop disables lazy loading and preloads the image, making it ideal for above-the-fold LCP (Largest Contentful Paint) images.
fill mode makes the image fill its parent container. The parent must have position: relative (or absolute or fixed) and defined dimensions.
The sizes prop tells the browser which image width to request at each viewport breakpoint. Without it, Next.js serves the full-size image to all devices.
Static imports (e.g., import hero from "@/public/hero.jpg") automatically provide width, height, and blurDataURL.
Remote images require width and height props (or fill) because Next.js cannot inspect them at build time.

Variations

Fill mode with aspect ratio container:

<div className="relative aspect-video w-full">
  <Image
    src="/images/banner.jpg"
    alt="Banner"
    fill
    sizes="100vw"
    className="object-cover"
  />
</div>

Responsive sizes for different layouts:

// Full-width hero
<Image src={src} alt={alt} fill sizes="100vw" />
 
// Two-column grid
<Image src={src} alt={alt} fill sizes="(max-width: 768px) 100vw, 50vw" />
 
// Three-column with sidebar
<Image src={src} alt={alt} fill sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw" />

Custom loader for external CDN:

import Image from "next/image";
 
const cloudinaryLoader = ({ src, width, quality }: { src: string; width: number; quality?: number }) => {
  return `https://res.cloudinary.com/demo/image/upload/w_${width},q_${quality || 75}/${src}`;
};
 
export function CloudinaryImage() {
  return (
    <Image
      loader={cloudinaryLoader}
      src="sample.jpg"
      alt="Cloudinary image"
      width={800}
      height={600}
    />
  );
}

TypeScript Notes

next/image exports ImageProps and StaticImageData types.
Static imports return StaticImageData with src, width, height, and blurDataURL properties.
The loader function is typed as ImageLoader from next/image.

import type { ImageProps, StaticImageData } from "next/image";
 
interface HeroProps {
  image: StaticImageData;
  alt: string;
  priority?: boolean;
}

Gotchas

Using fill without sizes forces the browser to download the largest image variant. Always pair fill with an appropriate sizes prop.
The parent of a fill image must have position: relative and defined dimensions. Without this, the image collapses to zero height.
placeholder="blur" with remote images requires a blurDataURL prop. Only static imports generate this automatically.
The priority prop should only be used on above-the-fold images (typically one or two per page). Overusing it defeats lazy loading benefits.
remotePatterns in next.config requires a server restart to take effect. Changes are not picked up by hot reload.
Setting width and height does not crop or resize the image element in the DOM. These values define the aspect ratio and request size. Use CSS for visual sizing.
Images from domains not listed in remotePatterns will fail with a 400 error at runtime, not at build time.

Alternatives

Approach	Pros	Cons
next/image	Built-in optimization, lazy loading, format conversion	Complex props, server-side dependency
Native img tag	Simple, no configuration	No optimization, no lazy loading by default
Cloudinary or Imgix	Advanced transformations, CDN delivery	External service, additional cost
@unpic/react	Framework-agnostic, works with multiple CDNs	No built-in optimization server

FAQs

What formats does `next/image` automatically serve, and how does it decide?

It serves WebP or AVIF based on the browser's Accept header.
The built-in Image Optimization API handles format conversion automatically.
No configuration is needed; it works out of the box.

When should you use the `priority` prop on an image?

Use priority only on above-the-fold LCP (Largest Contentful Paint) images.
Typically one or two images per page.
It disables lazy loading and preloads the image.
Overusing it defeats the lazy loading benefits.

Gotcha: What happens if you use `fill` without a `sizes` prop?

The browser downloads the largest image variant for all devices.
Always pair fill with an appropriate sizes prop to serve correctly sized images per breakpoint.

What CSS requirement does a parent element need for `fill` mode images?

The parent must have position: relative (or absolute or fixed).
It must also have defined dimensions (width/height or aspect ratio).
Without this, the image collapses to zero height.

How do static imports differ from remote images in `next/image`?

Static imports automatically provide width, height, and blurDataURL.
Remote images require explicit width and height props (or fill) since Next.js cannot inspect them at build time.
placeholder="blur" works automatically with static imports but needs a manual blurDataURL for remote images.

How do you configure `next/image` to allow images from an external domain?

// next.config.ts
const nextConfig: NextConfig = {
  images: {
    remotePatterns: [
      { protocol: "https", hostname: "images.unsplash.com" },
    ],
  },
};

A server restart is required after changes.

Gotcha: Do `width` and `height` props crop or resize the image in the DOM?

No. They define the aspect ratio and the requested image size from the optimization server.
Use CSS classes for visual sizing in the browser.

How do you use a custom loader for an external CDN like Cloudinary?

const cloudinaryLoader = ({ src, width, quality }: { src: string; width: number; quality?: number }) => {
  return `https://res.cloudinary.com/demo/image/upload/w_${width},q_${quality || 75}/${src}`;
};
 
<Image loader={cloudinaryLoader} src="sample.jpg" alt="Photo" width={800} height={600} />

What TypeScript types does `next/image` export for typing image props?

import type { ImageProps, StaticImageData } from "next/image";

StaticImageData is the return type of static imports, with src, width, height, and blurDataURL.
ImageLoader types the custom loader function.

What is the correct `sizes` value for a three-column grid with a sidebar?

sizes="(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 33vw"

This tells the browser to request full-width images on mobile, half on tablet, and one-third on desktop.

What error do you get for images from domains not in `remotePatterns`?

A 400 error at runtime, not at build time.
The image simply fails to load when the page is visited.
Add the domain to remotePatterns in next.config.ts and restart the server.

Next.js Image Optimization

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Next.js Image Optimization