Correct me if I am wrong, but Dreamstime appears to be a stock-photo catalog. That would be a good resource for someone who has the know-how to make their own cover, or for someone who's looking for images to give to their cover designer. It wouldn't be a very good resource for those of us that are very inept with photomanipulation software.
I've heard of people finding stock images off of Deviantart, but some artists also use that site as a web gallery, so not everything on the site is free use. I agree, though, that if you're careful with browsing, you might be able to find an artist through there.