How do I change the login screen background in Lion?
The easiest solution is to use "Deeper". You can download the beta version which works with Lion 10.7.4 (assuming your using Lion of course).

You can download it here. (Initiates download)

Authors website:


  Deeper is a personalization utility for Mac OS X which allows you to
  enable and disable the hidden functions of the Finder, Dock,
  QuickTime, Safari, iTunes, Login window, Spotlight, and many of
  Apple’s applications...


Example of option to modify login screen:



You can also revert your login screen back to it's default settings if you so choose.