LightDM login manager is much more useful and visually pleasing than the old GDM login, albeit with a fixed background wallpaper at present. Fairly soon, you will have the option for the LightDM login manager to automatically pick up the default wallpaper for any username highlighted at the login selection screen.
In the meantime, you can set the wallpaper to pretty much anything you want using another a magic setting in one of the LightDM configuration files. This applies in Ubuntu 11.10 and 12.04.
You wil have to launch a text editor with root privileges from the terminal (or via the run box using Alt+F2)
gksu gedit /etc/lightdm/unity-greeter.conf
Look down the settings until you find the line beginning background=
Change the background filename to anything you want to use, but do make sure you get the correct absolute path to the file name (with extension), for example:
needless to say, your choice of wallpaper needs to be accessible on a local filesystem that is already mounted when LightDM loads.
Save the file, log out to the LightDM greeter and see your glorious wallpaper! RC