The folks at Dual Screen Wallpaper provide desktop wallpapers that are optimized across multiple monitor systems. Be for warned though they have integrated the advertising in their site in such a way as to try to trick people into making false clicks; the titles and descriptions of each wallpaper are formatted exactly like the text ads that they display at the top of the screen, and they even display an image beside the text ads so that they blend in perfectly with the rest of the site. So, for now just be careful of the top two links on the site. The wallpapers are gorgeous and work beautifully on dual-screen setups.