I'd like to know where that dividing line is, exactly. Nowadays, 9 times in 10, a poster is a photoshopped and slightly "enhanced" collage of still images with some text on top. Remove that text and you have the "key art". But if the textless art is not to hand, why not just upload the original poster? Is the picture in the background somehow less valid, especially if it's rather rare, like some Oscar ads?
These aren't the only examples, but especially for the Harry Potter films people have been uploading collages, montages and artsy ads. You're also getting closer to fan art territory.