what about the ole' GameBoy's Mario land/wario land covers? they actually made me buy the games just because of the cover-art (sorry, no internet or reviews then for me)
Speaking of Psygnosis and Agony, I'm sure most people know, but if you didn't the artist for some of Psygnosis' art, and lots of their logos, is Roger Dean. http://www.rogerdean.com/ I've always liked his art since I was a child (My parents had a book of his art called "Views".) You see rip-offs of his art in games sometimes, too, like in the "cauliflower" alien trees in Cyber-Lip for the Neo-Geo.