Image not in a next-gen format

MediumDeveloper fixImagesPer-page checkresource_image_not_optimized

What this means

An image over 200KB is served in a legacy format rather than WebP or AVIF, which typically cost 25–50% fewer bytes at the same visual quality.

How to fix it

Convert the image to WebP or AVIF, keeping a legacy fallback only if you still support browsers that need one.

Fix effort

This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.

Authoritative source

https://developers.google.com/search/docs/appearance/google-images

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