Images not using modern format — the fix
One or more images use legacy formats (JPEG, PNG, GIF) instead of modern formats (WebP, AVIF) that offer significantly better compression at equivalent quality.
Where this fits: performance in SEO
Performance is both a ranking signal (Core Web Vitals) and a conversion multiplier. Google measures real-user loading (LCP), interactivity (INP), and visual stability (CLS) — and slow sites lose visitors before analytics even registers them. Most performance issues concentrate in a few repeat offenders: unoptimised images, render-blocking resources, and third-party scripts. Fixing the top three usually moves the needle more than fixing the next thirty.
Why images not using modern format hurts your rankings
WebP images are 25-35% smaller than equivalent JPEG images, and AVIF offers even greater savings. Since images typically account for the majority of page weight, converting to modern formats can dramatically reduce load times. Google PageSpeed Insights specifically flags images that could benefit from modern formats.
This is a low-severity issue: a polish item. Fix it in batches during scheduled maintenance; the win is cumulative quality, not a step change.
How to fix it
Convert images to WebP or AVIF format. Use the <picture> element to serve modern formats with JPEG/PNG fallbacks for older browsers. Most image CDNs and build tools can automate format conversion.
<picture>
<source srcset="image.avif" type="image/avif" />
<source srcset="image.webp" type="image/webp" />
<img src="image.jpg" alt="Description" />
</picture>Performance best practices
- Serve images in modern formats (WebP/AVIF), sized to their container, with explicit dimensions.
- Inline critical CSS and defer everything else; eliminate render-blocking third parties.
- Set far-future cache headers on static assets and an ETag on everything.
- Preload the fonts and hero image; font-display: swap to avoid invisible text.
- Measure with real-user Core Web Vitals, not just lab scores — Google ranks on field data.
The full library: SEO best practices, by category.
Frequently asked questions
What does "Images not using modern format" mean?
One or more images use legacy formats (JPEG, PNG, GIF) instead of modern formats (WebP, AVIF) that offer significantly better compression at equivalent quality.
Why does images not using modern format matter for SEO?
WebP images are 25-35% smaller than equivalent JPEG images, and AVIF offers even greater savings. Since images typically account for the majority of page weight, converting to modern formats can dramatically reduce load times. Google PageSpeed Insights specifically flags images that could benefit from modern formats.
How do I fix images not using modern format?
Convert images to WebP or AVIF format. Use the <picture> element to serve modern formats with JPEG/PNG fallbacks for older browsers. Most image CDNs and build tools can automate format conversion.
How serious is this issue?
This is a low-severity issue: a polish item. Fix it in batches during scheduled maintenance; the win is cumulative quality, not a step change. It belongs to the performance family of checks.
How do I find every page affected by this on my site?
Run a free Dr Urls audit: it crawls your site, detects images not using modern format on every affected page, shows example URLs, and generates a ready-to-use fix task. Re-scan after fixing to verify the issue is gone.
Does your site have this issue?
A free Dr Urls audit crawls your site, finds every page affected by images not using modern format, and hands you a ready-made fix task.
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