If you enabled WebP images in settings but still can’t see WebP version of the images in the source, please go to Image Optimizer and check that WebP has been generated for all images. If not, please enable Generate WebP in Swift Performance Settings » Images, and click to Optimize Images (All) in Image Optimizer.
If you set Serve WebP to <picture> mode, then you need to clear all cache after the WebP version has been generated for all images.
If you set Serve WebP to Rewrites then images will be served from the same URL (so you won’t see any change in the source), but the server will serve WebP version if the browser is compatible with WebP.
If you set Serve WebP to Rewrites please be sure that rewrite rules has been added to .htaccess. On Nginx server you need to add rules manually to Nginx config.