The default background image on OSX Leopard is this annoying Aurora/starfield kinda picture. To change this, just change the desktop background image by using the rightmouse button on the desktop.It's something different to change the background image on the login screen. Some searching revealed that it uses the DefaultDesktop.jpg in the /System/Library/CoreServices/ directory. 'Just' replace that file with your own picture. There is a small catch. If you replace it, it will get the wrong permissions on the file, and therefor won't show up on the login screen.Just 'reset' the permissions on the file like this:
- sudo chown root /System/Library/CoreServices/DefaultDesktop.jpg
- sudo chmod 755 /System/Library/CoreServices/DefaultDesktop.jpg