You can use the HypeResourceLoad event and in the handler redirect the image to a webp version. You would need to put these in your project library as well.
Also check out the Hype Cloud Image example (paid Image cruncher service with CDN) to atomically generate such images.
In that example you can see some code how to use HypeResourceLoad (although more in-depth than a simple solution for self hosted webp would need to be). The Event is also described in Hype Cookbook under Hype Events.