To optimize images in Next.js, first give each image the correct dimensions and responsive sizes, then decide whether the built-in Image Optimization API or an external image service should generate the image variants. Use a custom loader when an external service should handle transformations; the loader builds a URL from the source, requested width, and quality. The exact options vary by Next.js version, so check the documentation for the version installed in your project.
Start with responsive layout and image dimensions
Use next/image with meaningful alternative text and either intrinsic dimensions or a deliberate responsive fill layout. The width and height values describe the source aspect ratio and help reserve space while an image loads; CSS still determines its rendered size. Next.js explains that these attributes help avoid layout shift.
import Image from 'next/image'
export function ArticleImage() {
return (
<Image
src="/article.jpg"
alt="A person cycling along a coastal road"
width={1600}
height={900}
sizes="(max-width: 768px) 100vw, 50vw"
style={{ width: '100%', height: 'auto' }}
/>
)
}The example’s sizes value is illustrative, not universal. It says that the image is expected to occupy the full viewport width at widths up to 768 pixels and half the viewport width above that. Match these conditions and proportions to the actual CSS layout. For a responsive image, sizes helps the browser choose a suitable candidate from the generated srcset. Without it, the browser may assume the image is viewport-wide and download a larger candidate than the layout needs. See the Next.js Image Component documentation and the Pages Router Image reference.
Using fill
With fill, the image fills its positioned parent rather than taking an intrinsic width and height. Give the parent a defined layout size and positioning context, and choose whether the image should crop or fit inside that box.
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<div className="hero">
<Image
src="/hero.jpg"
alt="A mountain ridge at sunrise"
fill
sizes="(max-width: 768px) 100vw, 50vw"
style={{ objectFit: 'cover' }}
/>
</div>.hero {
position: relative;
aspect-ratio: 16 / 9;
}objectFit: 'cover' fills the box by cropping any overflow; use contain when the full image should remain visible, accepting any unused space. Adjust sizes to describe the width this box actually occupies at each breakpoint.
How do I use a custom loader with next/image?
A loader is a URL-building function. Next.js supplies the source, requested width, and optional quality; your function returns a URL in the format expected by the image service. The function does not itself perform image transformations or configure the external service.
Use a loader for one image
import Image from 'next/image'
const imageLoader = ({ src, width, quality }) => {
return makeProviderUrl({
src,
width,
quality: quality ?? 75,
})
}
export function ProductImage() {
return (
<Image
loader={imageLoader}
src="/products/shoe.jpg"
alt="Running shoe in blue"
width={1200}
height={800}
sizes="(max-width: 768px) 100vw, 33vw"
/>
)
}makeProviderUrl is deliberately provider-specific pseudocode, not a runnable integration. Replace it with the selected provider’s documented URL construction. Check how that provider expects the source to be encoded and how it handles allowed hosts, requested widths, quality, output formats, and transformations. The official Next.js loader example uses quality 75 when quality is omitted; confirm defaults and behavior against the installed version and chosen service.
Configure a loader for the project
For app-wide behavior, set images.loader to 'custom' and point images.loaderFile to a module relative to the project root. That file exports the default loader function.
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// next.config.js
module.exports = {
images: {
loader: 'custom',
loaderFile: './src/lib/image-loader.js',
},
}// src/lib/image-loader.js
export default function imageLoader({ src, width, quality }) {
return makeProviderUrl({ src, width, quality: quality ?? 75 })
}The provider URL function remains an integration you must implement. Consult the Next.js image configuration reference alongside the selected service’s documentation.
Built-in optimization or an external image service?
The default Next.js loader uses the Image Optimization API. A custom loader is the documented route when you want an external optimization service or image CDN to generate transformed image URLs. The right choice depends on deployment and source requirements; Next.js documentation establishes the integration route, not the features or costs of a particular provider.
| Decision point | Built-in Next.js optimizer | Custom loader and external service |
|---|---|---|
| Who generates image variants? | Next.js Image Optimization API | The external service addressed by the returned URL |
| URL behavior | Handled by Next.js configuration and its optimizer | Your loader must construct URLs in the provider’s documented syntax |
| Transformation support | Use the capabilities documented for your installed Next.js version | Depends on the provider and the transformations you encode in URLs |
| Cache and invalidation | Next.js optimizer cache behavior applies | Depends on the external service’s cache and invalidation behavior |
| Operational responsibility | Fits the Next.js deployment’s image optimization setup | Requires configuring, reaching, and maintaining the external service integration |
Before choosing, verify the service’s accepted widths and qualities, source-access rules, format negotiation, cache policy, and any costs that apply to your deployment. Those details are provider-specific. For the built-in optimizer’s format and cache behavior, see the Image Component API documentation.
How do I use a remote image with next/image?
When the built-in optimizer fetches remote sources, allow only the URL patterns your application needs. Configure remotePatterns with the expected protocol, hostname, path, and an intentional search-string policy. Avoid broad patterns: the Next.js documentation warns they can permit URLs that were not intended.
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// next.config.js
module.exports = {
images: {
remotePatterns: [
{
protocol: 'https',
hostname: 'images.example.com',
pathname: '/account123/**',
search: '',
},
],
},
}Replace the example host and path with the precise remote source you trust, and check the supported configuration syntax for your installed Next.js version. The default optimizer does not forward request headers when fetching an image. If the source requires authentication headers, the documentation advises considering unoptimized so the image is served without the optimization fetch.
SVGs are generally served unoptimized. If you enable SVG optimization, follow Next.js’s documented content-disposition and content-security-policy protections rather than treating arbitrary SVG sources as safe.
Format selection and caching in the built-in optimizer
The built-in optimizer selects from configured output formats based on the request’s Accept header. The documented default is WebP; AVIF can also be configured, with encoding and cache tradeoffs described in the Next.js documentation. If a proxy or CDN sits in front of Next.js, it must forward Accept for format negotiation to work as expected.
minimumCacheTTL sets a lower bound for the optimizer cache lifetime; the upstream image’s cache-control value can make the effective max age longer. The documentation states that there is no cache invalidation mechanism. If an image may change, avoid relying on a long cache lifetime for a URL that stays the same; use a changed source URL or clear the image cache when appropriate. These details apply to the built-in optimizer, not automatically to custom-loader providers.
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When should I preload an image?
Preload only when you know which above-the-fold image is likely to be the page’s Largest Contentful Paint (LCP) image. If that is not clear—for example, because the hero image changes with viewport—preloading candidates can waste bandwidth. The Pages Router reference notes that loading="eager" or fetchPriority="high" may be preferable in most cases. Follow the API supported by your installed version and coordinate preload with other loading or fetch-priority controls.
Troubleshooting image optimization
- The image is too large or slow to download: Check that
sizesmatches the rendered CSS width at each breakpoint, especially withfill. Inspect the generatedsrcsetand selected candidate in the browser. - The image causes layout shift: For intrinsic images, provide accurate
widthandheightvalues. Forfill, ensure the parent has a defined size and positioning context before the image loads. - A remote image is rejected: Verify that its protocol, hostname, path, and search policy match
remotePatterns. Narrowly configure the expected source rather than broadly allowing remote URLs. - An authenticated image fails through the default optimizer: The optimizer does not forward request headers to the source. Consider the documented
unoptimizedoption or a service integration designed for the source’s access requirements. - A custom-loader URL is invalid or returns an unexpected image: Check the provider’s URL syntax, source encoding, supported widths and quality values, host restrictions, and format behavior. A loader only constructs the URL; it cannot make an unavailable or incompatible service work.
- The output format does not change as expected: For the built-in optimizer, confirm that the format is configured and that any proxy or CDN forwards the browser’s
Acceptheader. With a custom loader, verify format selection with the provider. - Updated source content remains stale: Built-in optimizer caching has no invalidation mechanism. Change the source URL or clear the image cache as appropriate; external-provider cache controls are provider-specific.
- Preloading competes with more important resources: Preload only the known likely LCP image, and avoid combining it indiscriminately with eager loading or high fetch priority.
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