what was wrong with it?
Aye that's to force the game to export out as a different resolution than the default resolution that you set in game settings. It not recommended to use that - I think it's a bit buggy.
hmm... I think webp is supposed to work, but I recommend working directly with webp in the editor while you work on development rather than using the webp option when exporting - there's a reasoning behind my suggestion too, as it allows you to see how your game will run while working on your game rather than having to wait until you compile the game into a playable build.
If you have problems with building a game, backup (just in case) and delete everything in the tmp folder that will appear in you project folder.. that's were the webp images convert to.It might be an anti virus thing but for me Windows 10 doesn't overwrite the content of that folder meaning I end up with the old images and movies when I build a game.webp files appear to convert to that folder if you do it via the build option