From the not-sure-how-useful-this-is department comes a simple method of changing the default desktop wallpaper shown when logging into OS X Leopard if space and stars aren't your cup o' tea. All you need to do is replace the file DefaultDesktop.jpg located in /System/Library/CoreServices/DefaultDesktop.jpg. Since this is part of the System, you'll need administrative privileges to change the image.
Open up the Terminal in Applications » Utilities and type in the following:
sudo mv DefaultDesktop.jpg DefaultDesktop_org.jpg
sudo cp /Path/to/picture/you/want/to/use/image.jpg DefaultDesktop.jpg
After the first "sudo" command, you will be asked for your password. For the last line, instead of manually typing in the path of the image you want to use, you can just drag it into the Terminal. If you were to browse to the CoreServices directory in Finder after you did this, you'd see something like this showing the new image and the renamed old one.
and here are the fruits of your labor, a customized login wallpaper:
Alternatively, you may opt to go the "proper" route and add an entry into the appropriate plist file with (below is all one line to be run in Terminal):
sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/Path/to/picture/you/want/to/use/image.jpg"