art is by Imigi muru, the mangaka of "Kono Bijutsubu ni wa Mondai ga Aru" aka "this art club has a problem" which is a romcom that I personally recommend you to read.
lol. I don't use Photoshop but just reading this I'm convinced it's just Adobe being a little kid and jealous of Google for developing a good compressed format, otherwise adding support for it would be so easy for them that they could just add it in a point release and not even bat an eye (since webp is free and open and all that stuff)