Change Login-Screen-BG-Image at Leopard
found on: Change Login-Screen-BG-Image at Leopard
Don’t worry Tiger users, to accomplish this same thing you have to edit the following file:
/Library/Preferences/com.apple.loginwindow
Open Terminal and type
sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture '/Path/To/Picture.jpg'
Enter your password, log out and you should see the changes.