I've seen some pretty incredible edited images taken from Sims 3. It made me curious if anything like that would be allowed on this game and if so, how would it be credited. 

I believe that any screenshots from any games wouldn't be allowed. The game art and content is property of the game studio, it's not provided so that people can use it as stock unfortunately. There's some beautiful screenshots out there from beautiful games, but they can't be used as stock v-v Same rule applies to promotional art of actors/characters from shows/movies: the content is still copyrighted.