It's possible that nothing else can better set the mood for role-playing games excitement as good fantasy art. In a related fashion, role-playing can inspire the dabblers and professionals in fantasy art. The two passions go hand-in-hand.
Wizards of the Coast has some brand new fantasy art available free on their site, specifically formatted for desktop wallpaper.
With the release of the 4th Edition of the Player's Handbook, Dungeon Master's Guide, and Monster Manual, you also get glorious new art. We've chosen a few pieces to showcase here for you, and they've been formatted into handy-dandy desktop wallpaper sizes so that your computer doesn't feel left out of the 4th Edition fun. (Maybe you should send a link to the one depicting a noncombat encounter to remind your least cautious party member that blithely strolling through even an 'empty' dungeon isn't always a Good Idea.)
There are only nine so far, but it really gives a good idea of what artistic style can be found in the books. Check them out at 4th Edition D&D Core Rulebook Wallpapers.