Make sure to test things with the obvious caches disabled (Full Page Cache, Block HTML Cache). Once this extension is working, catalog images (like on a category page) should be replaced with: Their <img> tag should be replaced with a <picture> tag that lists both the old image and the new WebP image. If the conversion from the old image to WebP goes well.
You can expect the HTML to be changed, so inspecting the HTML source gives a good impression. You can also use the Error Console to inspect network traffic: If some webp images are flying be in a default Magento environment, this usually proofs that the extension is working to some extent.
This could mean that the conversion failed. New WebP images are stored in the same path as the original path (somewhere in pub/) which means that all folders need to be writable by the webserver. Specifically, if your deployment is based on artifacts, this could be an issue.
Also make sure that your PHP environment is capable of WebP: The function imagewebp should exist in PHP and we recommend a cwebp binary to be placed in /usr/local/bin/.
Last but not least, WebP images only work in WebP-capable browsers. The extension detects the browser support. Make sure to test this in Chrome first, which natively supports WebP.
Not all JPEG and PNG images are fit for conversion to WebP. In the past, WebP has had issues with alpha-transparency and partial transparency. If the WebP image can't be generated by our extension, it is not generated. Simple as that. If some images are converted but some are not, try to upload those to online conversion tools to see if they work.
Make sure your cwebp binary and PHP environment are up-to-date.
Don't complain to us. Instead, ask the other browser vendors to support this as well. And don't say this is not worth implementing, because I bet more than 25% of your sites visitors will benefit from WebP. Not offering this to them, is wasting additional bandwidth.
It could be that your transactional email templates are including XML layout handles that suddenly introduce this extensions functionality, while actually adding WebP to emails is a bad idea (because most email clients will not support anything of the like).
If you encounter such an email, find out which XML layout handle is the cause of this ({handle layout="foobar"}). Next, create a new XML layout file with that name (foobar.xml) and call the webp_skip handle from it (<update handle="webp_skip" />). So this instructs the WebP extension to skip loading itself.
error while loading shared libraries: libjpeg.so.8: cannot open shared object file: No such file or directory
Ask your system administrator to install this library. Or ask the system administrator to install WebP support in PHP by upgrading PHP itself to the right version and to include the right PHP modules (like imagemagick). Or skip converting WebP images by disabling the setting in our extension and then convert all WebP images by hand.