Images are served in older, heavier formats

LowHigh confidenceQuick winperf.lab.images-legacy-format

What this check looks for

‹count› image(s) are served as JPEG, PNG or GIF where AVIF or WebP would carry the same picture in far fewer bytes — an estimated ‹savings bytes› less on this page.

Why it matters

This is one of the few optimisations with no visual cost and no design decision attached: the same image, noticeably fewer bytes, on every visit.

When the check passes, your report says: “Images are served in modern formats such as AVIF or WebP”.

What it costs your score

When this check fails it removes 5 points from your Performance score, before the status, confidence and repeat multipliers are applied. Performance carries a weight of 8 in the overall score.

It shares the performance.assets family ceiling of 25 points: however many findings that family produces, together they cannot remove more than that from Performance. One underlying problem showing up in several places is still one problem.

Severity: low
Default confidence: high
Status when triggered: warn
Deduction: 5 points
Family cap: performance.assets · 25
Category: Performance
Module: Perf lab
Fix owned by: user
In the ruleset since: 2026.09

How the whole score is calculated

How to fix it

Serve images as AVIF or WebP, with the original format as a fallback.

The same images in a modern format would be an estimated ‹savings bytes› smaller with no visible difference.

Convert the images this finding lists to AVIF, and to WebP as a second option.
Offer them with <picture> and <source type> so the browser picks what it supports and falls back to the original file otherwise.
Keep the fallback. A <picture> with only AVIF sources and no <img> renders nothing where AVIF is unsupported.
If your CDN or image service supports format negotiation on Accept, turn it on — that converts every image at once, including ones added later.
Re-check quality at the new settings. AVIF at a given quality number is not the same picture as JPEG at that number; compare them side by side once and settle on a default.

How to confirm it worked

Re-run this scan and confirm the legacy-format count has dropped.
curl -sSI -H 'Accept: image/avif,image/webp,*/*' https://‹host›/path/to/image — check the content-type that comes back

A named slot like ‹domain› — and the braces left in the configuration below — is filled in with your own values when this rule appears on a report.

Remediation by platform

Page HTML

<picture>
	<source srcset="/photo.avif" type="image/avif">
	<source srcset="/photo.webp" type="image/webp">
	<img src="/photo.jpg" width="800" height="450" alt="">
</picture>

Order matters: the browser takes the first <source> whose type it can decode, so put AVIF before WebP. The <img> is both the fallback and the element that carries alt, width and height.

Technical detail

Images in legacy formats: ‹count›, estimated saving **‹savings bytes›**.

‹detail›

This came from **one page load we performed**, not from your visitors. A lab measurement is reproducible and tells you exactly which bytes and which resources are responsible — which is why the numbers below are byte counts and resource lists rather than a score. What real visitors experience is in the field-data section, which is the evidence this report scores Core Web Vitals on.

AVIF and WebP are supported by every current browser. AVIF generally compresses best, WebP is the safer floor, and <picture> lets you offer both with the original as a last resort — the browser picks the first source it can decode, so no visitor is left without an image.

Two caveats worth knowing before you convert everything. AVIF encoding is slow, which matters for a build pipeline but not for the visitor. And for very small images the format overhead can outweigh the compression win, so measure rather than assume — this finding lists the specific files where we measured a saving.