@pmoreno , According the docs of Grav on Supported Media Files, webp is currently not supported. Image manipulation functions like cropZoom will therefor not be supported.
Support for webp has been asked on the forum before in post WEBP Image format. A reply refers to the Github repo of Grav. On Github you will find issue WebP support missing #2714 (Oct 2019) and Grav’s future plans on webp.
Gravs response:
We currently have the same issue as #1168 - we are locked into a specific image library in Grav 1.x, and it doesn’t support WebP. With Grav 2.x we plan on revisiting this and updating to a more flexible library.
You might want to subscribe to that issue to get notified on any progress.