How to Fix: Images use WebP

Images use WebP is a high-priority check in the Site Performance & Core Web Vitals category, and it earns that priority honestly. On most WordPress sites, images are the single heaviest thing on the page. Serving them as modern WebP instead of legacy JPEG and PNG is one of the highest-leverage speed wins available — often 25–35% smaller files at the same visual quality, which shows up directly in your Core Web Vitals.

What this check actually means

The check looks at whether your images are served in a next-generation format — WebP in practice, with AVIF as the newer option — rather than as raw JPEG and PNG. A failure means your media library is delivering the old formats at full weight, which is the “serve images in next-gen formats” line you have probably seen in PageSpeed Insights. WebP was built for exactly this: it holds the same visual quality as JPEG or PNG in a noticeably smaller file, so the browser downloads less and the page paints sooner.

The reassuring part is that this no longer involves any format war. WebP is supported in every current browser, and the sensible setups serve WebP to browsers that accept it while keeping a JPEG or PNG fallback for anything that does not. You get the saving with no one left behind.

Why it matters for SEO

This one does touch rankings, through Core Web Vitals. Largest Contentful Paint — how quickly the main content appears — is very often gated by your hero image, and a hero image a third smaller loads a third faster. Google uses page experience as a ranking input, and more importantly, real users abandon slow pages, so the speed win is both an SEO signal and a conversion one. On mobile, where a lot of your traffic is on a middling connection, the difference between a bloated JPEG and a lean WebP is the difference between a visitor who waits and one who leaves.

How I would fix it in WordPress

Difficulty Level: EASY – This fix is straightforward and can typically be completed by anyone with basic WordPress knowledge.

Estimated Time: 15 min

You almost never do this by hand — you install a converter and let it work on the whole library:

1. Choose an image optimisation plugin that generates WebP with a fallback — the well-known options all do this and serve the right format per browser automatically.
2. Run a bulk conversion over your existing media library so past uploads benefit, not just new ones. On a big site this runs in the background; let it finish.
3. Set new uploads to convert automatically, so this stays solved instead of drifting back the moment someone adds a fresh image.
4. Combine WebP with the basics that multiply it — correct dimensions (do not ship a 4000px image into a 800px slot), lazy loading below the fold, and a CDN if you have one.
5. Re-test in PageSpeed Insights and confirm the “next-gen formats” flag has cleared and your LCP has moved in the right direction.

An old lesson about squeezing images

I have been fighting image weight since long before Core Web Vitals gave it a name. In the CD-ROM years I built multi-language projects where the entire experience — every screen, every asset, six languages of it — had to fit inside the hard ceiling of a disc, and there was no “just add more storage.” You learned to be ruthless about formats: which image could drop to a smaller palette, which could compress harder before the eye noticed, where a few kilobytes saved across hundreds of assets was the difference between shipping and not. Optimisation was not a nice-to-have. It was the constraint the whole project lived inside.

WebP is that same discipline, except the browser and a plugin now do the hard part for you, and the ceiling is your visitor’s patience instead of a 650MB disc. The instinct is identical: the smallest file that still looks right is the correct file. What used to take me an afternoon of hand-tuning per asset is now a bulk conversion you run once and forget. Do not leave that saving unclaimed — it was hard-won when I had to do it by hand, and it is nearly free now.

Recommended Tools & Plugins

These tools can help you fix and prevent this issue:

• Yoast SEO – Comprehensive SEO plugin with built-in checks
• Rank Math – Feature-rich SEO plugin with detailed analysis
• Google Search Console – Free tool to monitor your site’s search presence
• SEO Roadmap – Complete SEO audit and action plan tool

Need More Help?

If images still show as un-optimised after conversion, the usual causes are a caching layer serving old versions or images loaded by a page builder outside the media library that the plugin never touched — clear caches and check how your theme delivers images. For a large or performance-critical site, a developer can wire up WebP (or AVIF) delivery at the server or CDN level, which is faster and cleaner than doing it in PHP. A complete SEO Roadmap audit will show you which pages are heaviest so you fix the ones dragging your Core Web Vitals down first.

This guide is part of the SEO Roadmap knowledge base – your complete resource for WordPress SEO optimization.