What image do you have behind the login menu when you start up? Classically, depending on the exact version of Mac OS X you are using, you have one of the “universe” images. This image can be replaced on your system with a picture of your choice. To replace the image you see as the default desktop behind the login window, find a .jpg file that you want for your new back round.
Duplicate the image, preferably an image the same resolution as your machine default display resolution, and name the new file “DefaultDesktop.” Open a new finder window and navigate to (NameofyourHD) > System > Library > CoreServices. Drag and drop your new DefaultDesktop image into the Core Services folder.
When you release, you will be asked if you want to replace the existing file first and then to authenticate as an Admin User secondarily. The next time you restart and before you log in, you will see that image behind the login window.