I'm not at a Mac at the moment, so I can't give you the exact names, but try and locate a file called Aqua Blue.jpg. It should be in a folder called Desktop Images, and that folder is either in /Library or /System/Library. Can't remember which. Anyway, back up Aqua Blue.jpg just in case you want to use it later, then replace it with whatever you want.
The Aqua Blue.jpg image is the one that is used during startup and gets displayed behind the login screen. I've seen a lot of people add contact information to that image if they use a laptop. Helps to make the computer more identifiable.