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. Then I saved the AppleScript as an application, and added it to the login items for my account. Now every time I log in, I see a different login background. Since I really know nothing about AppleScripting, the above is probably not ideal (please feel free to suggest improvements in the comments), but it works for me.
[robg adds: I haven't tested this one, but to make it work, you'll also have to have used the linked hint to specify a new location for the login background images.]

