[ubuntu] Login screen has user background

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http://ubuntuforums.org – Since upgrading to 12.10, the login screen shows the desktop background of a particular user, rather than the generic background that I would expect. Is this a bug? Is there anyway to configure the desktop background to be used? I looked at the config files in /etc/lightdm, but I see nothing there that seems to imply using a user-specific desktop background for the greeter. In fact, unity-greeter.conf seems to be saying to use warty-final-ubuntu.png! (Hardware)