WebP is an image file format like JPEG or PNG but newer and better. WebPs look very sharp while being much more optimized in file size and are relatively efficient on slow devices.
WebP was released in 2010 in Google Chrome while PNG was released in web browsers in 1998 and JPEG goes back to the 1980s I think. Firefox didn't add support until about 2015 and Apple in 2020 because of a concern for its necessity and its possible costs for patents.
Many current e-book readers like the Amazon Kindle are running software and hardware that is similar to that of the Palm Pilots of the 1990s and so cannot handle the newer file formats like WebP and maybe still not even PNG or SVG. Officially though, WebP is now part of the EPUB standard as much as any other image format.
Most Apple Users
If you have Apple iOS 14 (on iPhones and iPads) or MacOS 11 or higher, you have complete support for WebPs including WebPs in EPUB (e-book) format. Those looking to open WebPs in iPhones, iPads, or Macs (not updated with free software (OS) updates since fall of 2020) can still see them by using the Zip file format version. If you are using an Apple device (iOS 14 or Mac OS 11 and higher) then you can just drag and drop or save the WebP version into an album in your photos library.
Other Apple Users
Apple iOS updated since fall of 2019 and Windows and Macs since around 2005 support opening Zip files without additional software. The individual pictures need to be opened with some App Store app for iPhone and iPad users (before iOS 14) that supports the WebP format. Pixelmator Classic and Pro have supported WebP for some time.
Windows and Other Users
These individual pictures can also be opened in Google Chrome, Firefox, or the latest Microsoft Edge Browser on any device that runs them except iPads and iPhones. Windows 10 users can also open them in Microsoft Paint. Android (and probably Linux) users should have even better support for this format than Apple or Microsoft users. Gimp supported WebP since 2.10. Nomacs, Okular, gThumb, Gwenview, and qview also support WebPs.