Quick answer
JPEG, PNG, and GIF are widely supported image formats, but modern formats such as WebP or AVIF can often deliver similar quality with fewer bytes. NOTABIS reports legacy-image-format when an <img src> path ends in .jpg, .jpeg, .png, or .gif. This is a low-priority opportunity, not proof that every image should be converted.
What the issue means
The filename extension hints at the encoded format. A legacy format may still be the right choice for transparency, animation, editing workflows, or compatibility. Modern formats need a fallback or responsive delivery plan when older clients or downstream tools require the original.
Why it matters for SEO
Smaller image transfers can improve loading and user experience, which supports a healthy page overall. The format itself is not a direct ranking switch, and conversion can reduce quality or remove useful metadata if done carelessly. Measure byte size and visual output rather than chasing an extension.
How NOTABIS detects it
The legacy-image-format rule scans img elements and reads the extension from the URL path in src. It counts .jpg, .jpeg, .png, and .gif case-insensitively and reports the total at page scope. It does not inspect srcset, CSS backgrounds, actual MIME bytes, compression quality, dimensions, or browser support.
How to check it yourself
Inspect image URLs in the DOM and Network panel, then check the response Content-Type, transfer size, dimensions, and visual quality. Compare a representative conversion in WebP or AVIF at the same display size. Test responsive srcset, transparency, animation, and fallback behaviour.
How to fix it
Convert suitable photographic or illustrative assets using an image pipeline that preserves dimensions and quality. Serve modern formats through <picture> or responsive srcset with a compatible fallback:
<picture>
<source srcset="shoe.avif" type="image/avif">
<source srcset="shoe.webp" type="image/webp">
<img src="shoe.jpg" alt="Red trail running shoe">
</picture>
Keep lossless PNG or GIF where transparency, pixel precision, or animation requires it, and recheck caching and alt text after migration.
When this is intentional or does not need fixing
Logos, screenshots, animated images, transparent assets, archival files, and third-party integrations may need a legacy format. A small image with no measurable transfer cost may not justify a conversion. Do not replace a working image solely to clear a low-severity audit finding.
Related issues
Review image alt text, missing image dimensions, and render-blocking resource candidates. Google’s image SEO guidance covers image context and alternatives.
Run a NOTABIS audit
NOTABIS lists image URLs using the formats covered by its extension heuristic so you can prioritise conversions with real size and quality evidence.