If you are searching for a modern image format test, you usually want one answer: which images on this URL are still shipping as JPG, PNG, or GIF when WebP or AVIF would cut the weight. That is not a theory problem. Images often make up most of the bytes on a marketing page, and legacy formats are one of the cheapest levers left once you have already poked at paint metrics.
Site Signals has a free check for exactly that. Paste a live URL into the Modern Image Format Test, run it, and see each image’s format, whether it is next-gen, and a practical sense of what converting would save.
What “modern image formats” means in practice
Modern image formats here means WebP and AVIF served to browsers that support them, with a safe fallback for everyone else. JPG and PNG still work. They also cost more bytes for the same visual quality on photos and most UI screenshots.
- WebP is supported across current browsers and typically lands about 25–35% smaller than a comparable JPG at matched quality.
- AVIF often saves more (commonly cited around 50% vs JPG on photo content) with support well above 95% of users when you keep a fallback.
- Animated GIF is the expensive outlier. A short motion clip as GIF can be many times larger than the same clip as MP4 or WebM.
Lighthouse and similar audits flag “serve images in next-gen formats” because the byte savings usually show up in Largest Contentful Paint when the LCP candidate is an image. Format alone is not a full performance strategy, but it is a concrete, measurable pass/fail you can fix in a build pipeline.
How to run the Modern Image Format Test
- Open the Modern Image Format Test.
- Paste a full https URL for the page you care about (homepage, product, landing page, or blog post).
- Run the free test. No account is required.
- Read the per-image list: format (JPG, PNG, GIF, WebP, AVIF, SVG), and which ones are still legacy.
- Optionally compare a competitor URL so you can see whether they already ship next-gen sources.
The tool walks image references on the page (including common <img> and <picture> sources), inspects what actually came back, and ranks what is worth converting first. Use that list as a punch list, not as a vanity score.
What a useful report looks like
A useful modern-format report is specific. You want URLs or file names, the current format, and enough signal to decide order of work:
- Hero and above-the-fold photos still on JPG or PNG
- Product galleries where every thumbnail is an unoptimized PNG
- Animated GIFs used as “video” in marketing sections
- Pages that already declare WebP or AVIF in
<picture>but still download a heavy JPG first - SVG icons mixed in (usually fine—do not “convert” vector UI chrome into lossy photos)
If the LCP element on the page is a large JPG hero, format conversion plus correct dimensions is often faster to ship than a redesign. If LCP is text or a background video, modern still images still help total weight and mobile data use, but they will not move that particular metric alone.
How to fix what the test finds
Treat conversion as a pipeline, not a one-off export in Photoshop.
- Convert at build or upload time. Use a library such as Sharp, Squoosh CLI, or your CDN’s image resizer so every new asset gets WebP (and ideally AVIF) automatically.
- Serve with
<picture>. Put AVIF first, then WebP, then a JPG/PNG fallback. That keeps older clients working without breaking the layout. - Replace animated GIFs. Prefer short MP4/WebM loops with a poster frame. Keep GIF only when you truly need frame-by-frame stills.
- Do not stop at format. Pair next-gen files with real width/height (or CSS
aspect-ratio), responsivesrcset, and lazy-loading below the fold. Format savings disappear if you still ship a 4000px image into a 400px slot. - Re-test the same URL after deploy. The Modern Image Format Test should show WebP/AVIF where JPG/PNG used to dominate.
Common mistakes that keep legacy formats winning
- Optimizing the homepage once, then uploading new blog heroes as full-size JPG forever
- Serving only WebP with no fallback, then discovering a broken image in an old corporate browser
- Exporting WebP at default quality that looks soft, then blaming the format instead of the quality slider
- Leaving marketing GIFs in the hero while the rest of the page is carefully compressed
- Fixing formats but ignoring render-blocking CSS/JS, so LCP still looks “Needs Improvement” for other reasons
If paint timing is still the bigger fire after formats look clean, run the related free checks for Largest Contentful Paint, First Contentful Paint, and render-blocking resources. Those answer different questions. The modern image format test answers only: are you still paying legacy image tax?
Quick checklist before you close the ticket
- Above-the-fold photos serve AVIF or WebP with a JPG/PNG fallback
- No animated GIF heroes in production
- Build or CDN pipeline converts new uploads without a manual step
- Image dimensions match the displayed size (no huge files in small slots)
- You re-ran the Modern Image Format Test on the live URL after deploy
Start with one important URL. Run the free modern image format test, convert the worst offenders, redeploy, and confirm the report flipped. That is a cleaner win than another generic “optimize your images” article with no URL attached.