Leopard: How to Change the Login Desktop Background

If you do not enjoy the login desktop background, you can change it. It is saved in /System/Library/CoreServices as DefaultDesktop.jpg. If you have root account permissions, you can rename any .jpg file as DefaultDesktop.jpg.
To perform the operation from your root account:

% cd /System/Library/CoreServices
% mv DefaultDesktop.jpg DefaultDesktop.jpg.org
% cp /Library/Desktop\ Pictures/Aqua\ Blue.jpg /System/Library/CoreServices/DefaultDesktop.jpg
% chown root:wheel /System/Library/CoreServices/DefaultDesktop.jpg
% chmod 644 /System/Library/CoreServices/DefaultDesktop.jpg

Think of applying back all permissions: -rw-r--r-- on root:wheel.
PS: do not perform the procedure if you do not feel confident with the terminal