For faster, more accessible pages, serve image files sized for the visitor’s screen, use AVIF or WebP with a fallback, reserve space with image dimensions, prioritize the main image people see first, and write alt text according to each image’s purpose. Lazy-load only images that are not immediately needed. Then check performance, accessibility, and whether important lazy-loaded content is discoverable.
1. Choose an efficient format and provide a fallback
AVIF and WebP can reduce image file sizes compared with many JPEG and PNG files, but the right choice depends on browser support, your image pipeline, and whether the image needs transparency or lossless detail. A practical default is to offer AVIF first, WebP next, and a JPEG or PNG fallback. The browser selects a supported source; the fallback remains available if it cannot use the modern formats. Google Chrome’s guidance describes this format negotiation pattern.
Use JPEG for photographs when a modern format is unavailable in your delivery pipeline, and PNG where its lossless characteristics or transparency are needed. Do not convert blindly: inspect the result for artifacts, especially around text, fine lines, gradients, and transparent edges. Choose a quality setting that preserves the image’s purpose rather than targeting the smallest possible file.
Use picture for format alternatives
The <picture> element lets you list format alternatives while keeping one fallback <img>. The browser considers the sources in order and uses the first one it supports.
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<picture>
<source type="image/avif"
srcset="hero-400w.avif 400w, hero-800w.avif 800w"
sizes="(max-width: 600px) 100vw, 50vw">
<source type="image/webp"
srcset="hero-400w.webp 400w, hero-800w.webp 800w"
sizes="(max-width: 600px) 100vw, 50vw">
<img src="hero-800w.jpg"
alt="A developer reviewing a page on a laptop"
width="800" height="600"
fetchpriority="high" loading="eager">
</picture>Ensure each listed file actually exists and depicts the same content. If a source URL fails, the browser may not automatically try another candidate from the same format list in every situation; monitor image delivery errors and keep your fallback healthy.
2. Deliver dimensions that fit the rendered image
An 1800-pixel-wide image is wasteful if it appears in a 360-pixel-wide column. Generate a small set of widths for each image and let the browser choose one based on the rendered size and display density. Responsive images are among the principal ways Lighthouse recommends avoiding delivery of more pixels than the page needs; an image CDN can automate resizing and format transformations. See Chrome’s properly size images guidance.
How srcset and sizes work
In a width-descriptor srcset, values such as 400w and 800w tell the browser the intrinsic width of each file. The sizes attribute estimates the image’s rendered width at different viewport sizes. The browser combines that estimate with screen density and its own loading decisions to select a suitable candidate.
<img src="article-800.jpg"
srcset="article-400.jpg 400w,
article-800.jpg 800w,
article-1200.jpg 1200w"
sizes="(max-width: 600px) 100vw,
(max-width: 1000px) 80vw,
700px"
alt="A close-up of the finished wooden chair"
width="1200" height="800">This example says that the image uses the full viewport width up to 600 pixels, about 80% of the viewport up to 1000 pixels, and otherwise occupies a 700-pixel slot. Adjust those estimates to match your actual layout. If sizes claims a much wider slot than the design uses, browsers may download unnecessarily large files; if it claims a narrower slot, the image may look soft on high-density displays.
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Build or automate the image variants
You can generate variants during your build and maintain the file mapping yourself, or use an image CDN to resize and transform source files on request. Build-time processing gives you control and predictable output, but requires a workflow for generating, naming, and updating variants. A CDN reduces that manual work, though it introduces a third-party delivery dependency and its own configuration. MDN names Squoosh as a web app for converting images to WebP and AVIF: MDN multimedia images guidance.
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3. Prevent images from shifting the page
Set the intrinsic width and height on each <img>, even when CSS makes it responsive. The browser can calculate the image’s aspect ratio before the file arrives and reserve the right amount of space. Without that reservation, text and other content may jump when the image loads, contributing to cumulative layout shift (CLS).
<img src="product.jpg"
alt="Blue ceramic mug viewed from the front"
width="1200" height="900"
style="max-width:100%; height:auto">The dimensions should describe the source image’s actual aspect ratio, not the display size you happen to want. For art-directed images with different crops at different breakpoints, use dimensions and CSS that reflect the selected presentation. If image dimensions are not practical, reserve a stable box with CSS aspect-ratio and an appropriate fit rule.
Test on slow connections and narrow screens as well as on a desktop. A layout that appears stable after the image has loaded can still shift noticeably during the initial load.
4. Load the important image first—and defer the rest
Do not lazy-load an above-the-fold image or the page’s likely Largest Contentful Paint (LCP) image. The browser needs to discover and fetch that image promptly. Use normal eager loading for a critical image and consider fetchpriority="high" when it is genuinely the main visual resource. Avoid assigning high priority to many images: they would compete with one another and with other important page resources.
For images well below the initial viewport, native loading="lazy" lets the browser defer fetching them until they are near being needed. This can reduce early network work and shorten the critical rendering path. MDN describes native lazy loading and its use for noncritical resources at MDN’s lazy loading guide.
Rank #3
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<!-- Main image visible when the page opens -->
<img src="feature.jpg" alt="The completed garden plan"
width="1200" height="800"
loading="eager" fetchpriority="high">
<!-- Supporting image farther down the page -->
<img src="details.jpg" alt="Close-up of the planting labels"
width="800" height="600"
loading="lazy">Do not add loading="lazy" indiscriminately to every image. It can delay an image needed for the initial view, including an LCP candidate. Conversely, eagerly loading a long gallery asks the browser to fetch content a visitor may never reach.
Keep lazy-loaded content discoverable
Important images should load when they become visible without requiring a click or other interaction. Google Search does not scroll or click through a page as a person would, so content that only appears after such interaction may not be discovered. Follow Google’s recommendations for fixing lazy-loaded website content: ensure content enters the viewport through normal scrolling and use mechanisms that Google can trigger or observe.
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Alt text is a text alternative, not a caption or a place to repeat keywords. Ask what information or function the image contributes in its current context. W3C WCAG 2.2 Success Criterion 1.1.1 states: “All non-text content that is presented to the user has a text alternative that serves the equivalent purpose.” See the WCAG 2.2 standard.
| Image purpose | What to put in alt | Example |
|---|---|---|
| Informative | A concise equivalent for the meaningful information the image conveys. | alt="Line chart showing monthly sign-ups rising from January to June" |
| Decorative | An empty value, alt="", so assistive technology can skip it. |
alt="" |
| Functional link or control | The action or destination, rather than a visual inventory of the image. | alt="Search" for a search button represented by an icon |
W3C’s Images Tutorial explains these informative, decorative, and functional categories. For an informative image, keep the alternative short while including text embedded in the image if that text is important to understanding it. W3C’s H37 technique describes providing a short text alternative with the alt attribute.
Context matters. A photo used as a link may need to describe the link’s destination or action, not every visible detail. If nearby text already communicates the same information and the image adds nothing, a null alternative can prevent needless repetition. Avoid using a filename, “image of” as a default opening, or a string of search terms.
Rank #4
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6. Validate speed, accessibility, and behavior
Check more than whether an image looks crisp on your own screen. Google’s good Core Web Vitals targets are LCP within 2.5 seconds, INP below 200 milliseconds, and CLS below 0.1. These are page experience metrics, not image-only scores; images can affect LCP through loading time and CLS through unreserved space. Google explains the metrics and thresholds in Understanding Core Web Vitals and Google search results.
- Inspect delivery: confirm the selected file format, transferred size, and dimensions at mobile and desktop viewport sizes.
- Check the initial view: make sure the main image is not delayed by lazy loading and does not wait behind avoidable work.
- Watch layout stability: observe the page while images load, especially on a slow connection.
- Review accessibility: verify informative, decorative, and functional images have appropriate alternatives.
- Test scrolling and indexing behavior: ensure lazy images appear as they approach the viewport and do not depend on clicks.
- Recheck after changes: a new crop, font, container width, or content order can change which image is largest or visible first.
MDN cites imagery as 51% of bandwidth for the average website in its multimedia guidance; treat that as a reported aggregate, not a prediction for every site. It also cites historical median image resource weights rising from approximately 250 KB to 900 KB on desktop and 100 KB to 850 KB on mobile between 2011 and 2019. Those figures describe historical context, not current universal benchmarks. See MDN’s image guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. Troubleshoot common image problems
Images look blurry on a phone
Check that srcset includes a candidate large enough for the rendered width and device pixel density, and that sizes reflects the real layout. Verify that the browser is not being offered only a small source and that your CDN or build pipeline has not generated a low-resolution variant.
The page jumps when an image appears
Add accurate intrinsic width and height, or reserve the correct aspect ratio with CSS. Also check that responsive crops maintain a reserved ratio at every breakpoint.
The main image appears late
Remove lazy loading from the image visible at page open. If it is the LCP image, use eager loading and consider a high fetch priority for that image only. Check whether CSS backgrounds or JavaScript are hiding the image URL from early browser discovery.
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A below-the-fold image never appears
Confirm its URL returns a valid image, the element has nonzero dimensions, and no script error prevents loading. Test by scrolling it into view. If search discovery matters, do not require a click to reveal it.
A modern format fails for some visitors
Check the type on each <source>, ensure the encoded file is valid, and keep a working JPEG or PNG in the fallback <img>. Inspect network responses for incorrect MIME types or missing assets.
Screen readers announce unhelpful text
Review each image in context. Replace filenames and redundant descriptions with a concise equivalent for informative images, use alt="" for purely decorative ones, and describe the action for icon buttons or linked images.
Or skip the browser setup
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