the "standard" login screen background on OSC (Leopard and Snow Leopard) can be modified using these command line incantations:
cd /System/Library/CoreServices
sudo mv DefaultDesktop.jpg DefaultDesktop_org.jpg
sudo cp /path/of/image.jpg DefaultDesktop.jpg
I learned that here
QUESTION: is there a GUI based utility for people like my wife and daughter?
.. they DO want to customize (worse: they want me to do it ),
but they DON'T want to learn the terminal ( since I know how).
They DO know how to enter an admin's credentials when prompted.
Best Answer
There's this that will match the login screen to the current wallpaper.
And this to set it to the image you like.
I haven't tried any of those, I did with the CLI, too.