Change LightDM's login screen wallpaper via Ubuntu Tweak in Ubuntu 11.10

Days ago, the handy tool filled with numerous tweak options, Ubuntu Tweak 0.6, was released.
LightDM displays, at login, the Unity greeter that is based on Ubuntu's default wallpaper and, while it can be changed by modifying a text file, Ubuntu Tweak 0.6 offers an easier alternative.

If you want to change the login wallpaper, open Ubuntu Tweak-->Tweaks-->Login Settings (take a look here for how to install it), then

unlock the tool

Set the same background as the current desktop background doesn't quite work (probably a bug), nevertheless changing the login background is as easy as clicking on the preview and selecting a background.

The important part is to choose only a wallpaper from /usr/share/backgrounds (otherwise it doesn't work).

If you desire to choose a particular wallpaper that isn't provided by Ubuntu, copy it to /usr/share/backgrounds and repeat the above method.