I learned about the general premise of making my own dual-screen wallpapers from videojug. Below are some wallpapers I use on a regular basis. I use Photoshop or GIMP and the source file is set to 300dpi. That way I can resize images without losing quality. The final product is reduced to 72dpi. You can leave it at 300dpi if you want. I did that so I can share the files easier.
Dual-screen Gallery
GTA IV
Niko Bellic from Grand Theft Auto IV. [1256x1024 pixels]
Transformers
The two opposing forces in the world of Transformers. [2560x1024 pixels]
Mass Effect 2
Concept art from Mass Effect 2 [2560×1024 pixels]
Saren Arterius from Mass Effect
Saren Arterius, a former turian Spectre, from Mass Effect [2560×1024 pixels]
Ken vs Ryu
The infamous duo, Ken and Ryu from Super Street Fighter IV. [2560×1024 pixels]
Street Fighters
Some of the characters in the Street Fighter universe. [2560×1024 pixels]
Street Fighter IV
A look at the core fighters of Super Street Fighter IV. [2560×1024 pixels]
Pathfinder RPG
An inverted view of a classic PFS scenario. [3840×1200]