One of my favorite sites for mapping art (and character & monster tokens) is www.rpgmapshare.com. I was surprised there wasn't already a link to it. There is art on there for fantasy, modern, sci-fi, and more.
And if you like this sort of thing, the Dundjinni user art forums are even more impressive.
Looking for cross-platform virtual tabletop software that can be used with any RPG system?