Change Login Image OSX

Change your Login background Image with a single "Drag-and-Drop" with this tool
http://www.loginox.branox.com/

Or you can do it 'manually' by using the terminal:
Find a desktop wallpaper image, save it to your desktop as background.jpg, and in the Terminal enter:
sudo mv /System/Library/CoreServices/DefaultDesktop.jpg /System/Library/CoreServices/DefaultDesktop.jpg.old

You'll have to enter your password to complete the operation. Then, in your Desktop directory in the Terminal, type:
sudo cp background.jpg /System/Library/CoreServices/DefaultDesktop.jpg

Now, next time you log off, your logon screen will have your new custom background. To undo the operation, in the Terminal, type:
sudo cp /System/Library/CoreServices/DefaultDesktop.jpg.old /System/Library/CoreServices/DefaultDesktop.jpg

Alternately, you can modify your logon background by changing a preference versus overwriting the file.
Here's the command:
sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/Library/Desktop Pictures/Aqua Blue.jpg"Substitute "/Library/Desktop Pictures/Aqua Blue.jpg"
with the path to the picture you want to use.
This technique works in both Leopard and Tiger.