<ramble>I would definately say that photographs are a great source of inspiration for my building. I think there is a difference between photos and art though. I don't think you could really get details and things from paintings as they may not be completely accurate, but then again some paintings are accurate, and you might not even want a completely accurate train anyway. Sure, some photography could be classified as art, but how many details of a train are you going to get from a silhouette in the sunset or something? I think it is a bit too broad of a question to ask if I use paintings etc. as inspiration for MOCs, I think I just use any sort of media depending on if I like the look of it or not.</ramble>
EDIT: I just saw the image in your post TheBrickster, and I suppose that is a different matter, that image could certainly inspire a diorama.