I really this older hint, on how to change the login window background by telling the system to look for the image in another location, but I want to suggest a small variation of it.
I like to constantly change my backgrounds. I have a directory with a bunch of background pictures, so I wrote a simple AppleScript which takes a random file from this directory and saves it as the picture used as the login window background. Mine is named myloginwindow.jpg, and it sits in my Public folder.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071231051934763