This set of instructions is written for Mac OSX version 10.5.2.
First, run this command in terminal to back up your default photo, just in case you ever need it again.
sudo mv /System/Library/CoreServices/DefaultDesktop.jpg /System/Library/CoreServices/DefaultDesktopBackup.jpg
Then, run the following terminal command to copy the new background to the system library. In this example below, we assume newBackground.jpg is the photo of your choosing.
sudo cp newBackground.jpg /System/Library/CoreServices/DefaultDesktop.jpg
The next time you login, you will see your new photo replacing the default Mac one.
If you decide you wish to revert this change, just run the following terminal command to copy the default image back.
sudo cp /System/Library/CoreServices/DefaultDesktopBackup.jpg /System/Library/CoreServices/DefaultDesktop.jpg