Since Ubuntu 11.10 GDM is replaced by LightDM. The LightDM login screen background can be changed by using the following tool: Simple LightDM Manager.
How to install Simple LightDM Manager:
To install this tool, the following steps needs to be executed:
Simple LightDM Manager can be found by searching from Dash home. The tools is limited to changing the background image and the logo.
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