For Leopard Users
To change the background, just replace the picture file located here:
/System/Library/CoreServices/DefaultDesktop.jpgBe sure to keep the original file in case you want to change it back to the original picture. We suggest changing the original to "DefaultDesktopOriginal.jpg" or something similar.
For Tiger Users
Tiger users don't have it as easy. You have to replace the picture by editing a login window preference file. Open Terminal and type in the following command, replacing "path_to_picture.jpg" with the actual path to the picture you want as the background:
sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture 'path_to_picture.jpg'
Since this is a sudo command, you will be required to authenticate as an administrator before the command will be run. Once you enter the command and logout, you should see the changes.
Want more tips and tricks like this? Visit TUAW's TUAW Tips and Terminal Tips sections.