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Start with the image’s content and required capabilities
There is no single “best” format for every website image. A photo, a logo, a screenshot and an animation have different compression and rendering needs. Decide what the asset contains and what it must support before choosing an extension.
| Asset or requirement | Good starting format | Why and what to watch |
|---|---|---|
| Logos, icons, diagrams and geometric illustrations | SVG | It scales without raster blur. Give the vector an appropriate accessible name or mark it decorative when used through an img element. |
| Photographs and photo-like editorial images | JPEG, lossy WebP or lossy AVIF | Lossy coding removes detail that is difficult to notice in photographs. Compare files at their rendered size and inspect faces, gradients and fine textures. |
| Screenshots, UI captures, diagrams and line art | PNG, lossless WebP or lossless AVIF | Sharp text and straight edges reveal ringing and blur quickly. Use lossless encoding or a carefully tuned quality level. |
| Transparency | PNG, WebP or AVIF | All three can preserve transparent pixels; choose the smallest option that works in your target browsers or embedded platforms. |
| Animation | Animated WebP or AVIF; video for longer sequences | GIF is limited to 256 colors and can be much larger than modern formats or video. |
| Legacy or broad fallback | JPEG or PNG | They remain widely supported raster fallbacks when a browser, WebView or publishing system cannot use a modern format. |
SVG is not a replacement for every raster image
SVG is ideal when the source is made from paths, shapes and text-like geometry. It is usually the wrong tool for a camera photograph. Keep text as real HTML whenever possible rather than baking essential words into a bitmap or vector image; real text is searchable, selectable and zoomable.
WebP versus AVIF
WebP and AVIF generally compress better than older formats, so they can reduce transfer size at comparable visual quality. AVIF can be especially efficient, while WebP is often a simpler operational choice. The correct winner depends on the image, encoder settings, decoding environment and the fallback work your audience requires. Export both, inspect them at the final display size, and measure bytes rather than choosing from the file extension alone.
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How JPEG, PNG, WebP and AVIF differ in practice
Lossy and lossless encoding
Lossy compression can make a photograph dramatically smaller, but it discards information. That trade-off is often acceptable for natural photographic detail. Screenshots, diagrams, logos and line art contain high-contrast edges and small text; aggressive lossy compression creates halos, ringing and fuzzy characters around them. Start with lossless PNG, lossless WebP or lossless AVIF for those assets, then test whether a visually equivalent lossy setting is acceptable.
Transparency and backgrounds
JPEG does not carry an alpha channel, so it cannot represent a transparent background. PNG, WebP and AVIF can. If an icon must sit over changing page colors, keep transparency; if it always sits on a known solid background, flattening it may reduce bytes.
Fallback strategy
Use a picture element when browser, CMS, email or embedded-WebView support is uncertain. The browser selects the first supported source and falls back to the nested img:
<picture>
<source srcset="hero.avif" type="image/avif">
<source srcset="hero.webp" type="image/webp">
<img src="hero.jpg" alt="Description of the subject and purpose" width="1200" height="675">
</picture>Do not remove the fallback merely because modern desktop browsers support AVIF. A controlled audience may still include older browsers, native WebViews, crawlers, image-processing tools or editorial systems with narrower support.
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Choose dimensions from the layout, not the source file
“What size should a website image be?” has no universal pixel answer. Begin with the maximum CSS width at which the image will render. A 2,400-pixel camera export displayed at 600 CSS pixels wastes transfer unless a high-density display genuinely needs that detail. Conversely, a 600-pixel source stretched across a 1,200-pixel hero will look soft.
- Measure the rendered slot. Inspect the CSS at the widest breakpoint and record the image’s maximum CSS width.
- Account for device pixel ratio. Provide a larger candidate when sharpness on high-DPR screens matters, but do not ship a source substantially wider than its intended use.
- Create practical width variants. Include candidates around your layout breakpoints, such as 480, 800, 1280 and 1920 pixels, adjusting the set to your actual designs.
- Keep the aspect ratio consistent. Use the same crop for ordinary responsive scaling. Create separate art-directed crops only when the composition must change on mobile.
- Reserve layout space. Set intrinsic
widthandheightattributes so the browser knows the aspect ratio before the file arrives.
Use srcset and sizes together
srcset lists candidate files and their intrinsic widths. sizes tells the browser how wide the image is expected to be in the layout; it does not change the rendered CSS size, which remains the job of CSS.
<img
src="hero-1280.jpg"
srcset="hero-480.jpg 480w, hero-800.jpg 800w, hero-1280.jpg 1280w, hero-1920.jpg 1920w"
sizes="(max-width: 700px) 100vw, (max-width: 1200px) 80vw, 1200px"
width="1200"
height="675"
alt="Description of the subject and purpose">The browser combines the slot estimate, viewport and device capabilities to select a candidate. Without accurate sizes, it may assume the image occupies the full viewport and download a needlessly large file.
When a hero needs art direction
A desktop landscape crop may place the subject too far from the focal point on a narrow screen. Use picture with different crops when composition, not just resolution, must change. Keep the alternative text focused on the image’s purpose rather than describing every crop.
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- Classify the source: photograph, screenshot, vector artwork, transparency-heavy graphic or animation.
- Select an initial format: use the decision matrix, then prepare a fallback if your audience or platform needs one.
- Resize before encoding: generate the largest actual display width first. Encoding an oversized original does not improve a smaller rendered image.
- Compare quality settings: review each candidate at its real CSS size and on a high-density display. Check text, edges, gradients, faces and transparent boundaries.
- Generate width variants: produce the candidates used by your layout and connect them with
srcsetandsizes. - Measure representative pages: test realistic devices and network conditions, paying special attention to the hero or other likely Largest Contentful Paint image.
Stop reducing quality when artifacts become visible. A smaller file is not an improvement if readers can see blurred text, blocky skies or halos around objects.
Make the browser’s loading work predictable
Reserve space to prevent layout shifts
Set dimensions on every content image, or use CSS with a known aspect ratio. This lets the browser reserve space while the request is in flight and prevents text from jumping when the image arrives.
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Prioritize only genuinely important images
A hero that is the likely Largest Contentful Paint element deserves early attention; below-the-fold images should not compete with it for initial bandwidth. Keep the file no larger than the slot requires and avoid loading multiple format variants in a way that downloads more than one.
Do not lazy-load the first viewport by habit
Lazy loading is useful for images far below the initial view, but applying it to the main hero can delay the page’s most important visual. Base the decision on the actual layout and performance measurements.
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Write useful alternative text
For an informative image, describe the subject and its purpose in the page. A product diagram may need the relationship it explains; a decorative flourish usually needs no words. Use an empty alt="" for decorative images so assistive technology can skip them.
Keep essential prose out of images
Text embedded in an image is not selectable, searchable or reliably zoomable. Put headings, instructions and labels in HTML whenever possible, and use the image only for information that genuinely requires a visual.
SVG still needs an accessibility decision
Embedding a vector through img does not automatically communicate its purpose. Supply meaningful alternative text for an informative logo or illustration, or an empty alternative for a decorative mark.
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Troubleshooting common image problems
The image looks blurry on a phone
Check the rendered CSS width, available candidates and sizes. Add a candidate large enough for the device’s pixel ratio, but keep it near the largest real display need.
Text in a screenshot has halos or ringing
Switch from aggressive JPEG or lossy WebP/AVIF to PNG or a lossless modern format, or raise the quality setting. Evaluate at the actual display size rather than zooming far beyond it.
The page downloads a desktop-sized file on mobile
Verify that srcset uses width descriptors such as 480w, that sizes matches the CSS layout, and that the server is not rewriting every candidate to the same URL.
Transparency appears as a solid color
You are likely serving JPEG or flattening the alpha channel during conversion. Use PNG, WebP or AVIF and confirm that the delivery pipeline preserves transparency.
Modern formats fail in an embedded browser
Keep the JPEG or PNG fallback inside picture, and test the actual WebView or publishing tool rather than relying on a desktop browser’s support.
The layout jumps while images load
Add accurate width and height attributes or an aspect-ratio rule. Make sure responsive crops retain the intended ratio or declare the art-directed dimensions.
A practical decision checklist
- Is the source geometric artwork? Start with SVG.
- Is it a photograph? Compare JPEG, WebP and AVIF at the target size.
- Does it need transparency? Use PNG, WebP or AVIF.
- Does it contain tiny text or hard edges? Prefer lossless encoding or conservative quality.
- Could your audience include constrained browsers or WebViews? Add a JPEG or PNG fallback.
- Have you measured the maximum CSS width and generated sensible candidates?
- Are
srcset,sizes, intrinsic dimensions and CSS consistent? - Does the alternative text communicate the image’s purpose?
- Have you checked the hero image under representative network and device conditions?
Frequently asked questions
Is PNG or JPG better for a website?
Neither universally wins. JPEG is usually more efficient for photographs without transparency; PNG is safer for transparency, screenshots and precise edges. Test WebP or AVIF as modern alternatives.
How many pixels should a hero image be?
Make the largest candidate match the hero’s maximum rendered CSS width, with an additional high-density option when needed. Derive the number from your layout rather than a universal rule.
Should every image be AVIF?
No. Format compatibility, decoding behavior, transparency, animation and the visual result still matter. Use AVIF where it is smaller at acceptable quality and provide a fallback when your platform requires it.
Does sizes resize an image?
No. It describes the expected layout width so the browser can choose a srcset candidate. CSS controls the displayed size.
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