Ahh, Digital Blasphemy! I love that site!! ALL of my PC wallpapers come from there.
If you pay for the membership ($25 a year I think) you can access all of their photo's AND the section where you can pick the size you want.
Each of their pictures are done in about 30 different sizes. You can even download their entire collection for a set size in one zip folder (I have the files for 1920x1200 for my PC, 960x800 for my Incredible and the 480x800 for the Incredible lockscreen).