From what I understand, Windows 8 will have a feature that will let you display two different desktop images (wallpapers) on your dual monitor setup. However, there are at least two ways you can accomplish the same in Windows 7 (and possibly other versions of Windows, but I haven’t checked).
1. Use a custom-sized image and set it to “tile” rather than “center”.
You can download custom-sized wallpapers for multiple monitor setups here and here (and many other places — just search). Right click on the full-sized image (make sure it is the full-sized image and not just a thumbnail) and select “tile” instead of “center” for the layout of the image.
If you want to use specific photos, you can figure out the dimensions of your two monitors (e.g. 3360 x 1050 — you can find this out in your display settings) and create an image of that size, and then tile it.
2. Download a program that will sort it out for you.
I haven’t tried either of these, but they seem to be reputable.
Lifehacker also has some hints of ways to make the most of your dual monitors.