Did you do this with the Terminal or Third party app?
I would use a third party program to do this or the library preferences listed bellow, as the area you alluded to is a dangerous place to make a typo.
sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/path to the picture you want to use/"
(you will have to enter an admin password at the end)
There is another way through terminal using sudo mv but I don't recommend it as a typo in the System/Library/CoreServices area is a bad bad bad thing.
Let me know how this works out for you.