thanks, I used gimp to convert from .webp to .png. I guess the plasma desktop does not support .webp images as wallpapers then? It would be cool to support them in the future.
Check if you have qt5-imageformats package installed. It is an optional dependency for kio-extras package that is not installed by default on Arch distros.
I am pretty sure webp works with the wallpaper picker on plasma5 from my experience.