I was browsing some high resolution dual & tri screen monitor wallpapers at
http://wallpaper.panedia.com/ and I noticed something really cool. They looked at my user agent string and resolution settings to determine which wallpaper was best for me.
Considering that (without 3rd party add ons) Windows can't display more than 1 image on multi-monitor setups, you have to choose 1 large image. They were nice enough to highlight the correct resolution as well. Kudos to Panedia for that!
(note: there were a ton of sizes available, my highlighted suggestion was way at the bottom)