I saw there were some wallpapers out of the DotA 2 art put together, however my screen has a massive resolution and they were too small for it.
I just put together the four pictures and kept them original size, and thought I'd share it.
For best fit, resize the width of the wallpaper to the width of your screen (resolution wise) and constrain properties so the height is resized by the same proportion, and set is as "Centre" mode (as opposed to stretched, tiled)
If you're using Windows 7 (Maybe vista too) just select the 'Fit' option in the wallpaper settings thing.
Full size is 3720 x 1600