You know, they advertised G4U to use on your PS3 wallpaper, and it extended out to computers and etc. etc.
Couldn't you just download one uploaded by someone else and put it on your PS3? I mean...my PS3 was primarily used by my brother and since he moved out it (more or less) collects dust so I don't know all its ins and outs, but you can use external pictures for wallpaper on a PSP for sure...
like, if you can upload pictures from G4U, I'm sure someone's going to upload a ton of them, and then everyone has free G4U wallpapers except the guy who uploaded 'em...
then again, I could be taking it too lightly XDD