One possibility is the .webp format? I import most of my stuff into FoundryVTT with that format, using TheRipper93's Media Optimizer module, and it's been working fairly well for me. I know this is late, but thought I'd mention it!
I tried webb format but unfortunately it can only work with smaller maps (less than 50x50 tiles) any other gives an error "the resolution is too high for webb format"... :(
