One thing to keep in mind when you design your desktop backgrounds is that it is your desktop. So, have fun with it and don't limit your possibilities. But also keep in mind that the desktop is the launching area and workspace for the applications you run in Windows. You need to have a background that does not obscure the icons on the desktop.
You might also think about choosing background colors and designs that are not hard on your eyes. The desktop is the biggest and most pronounced part of the Windows interface. So, you need one that is easy on your eyes but also blends well with the rest of your interface skin.