A default install of Ubuntu will start lightdm (the Light Display Manager) and uses the Unity greeter for user login. The default background can be changed to any image you like by modifying the Unity greeter configuration file. This can be done with the following command (from a terminal):
When gedit opens with the file contents there will be an section [greeter] and below that will be a key-value pair named "background". Just assign it to the full path of the image you wish to use, for example:
On my system in my home folder I have a ".wallpapers" folder where I store my wallpapers to use. "Current.jpg" is actually a system link to the wallpaper that I want to use, such that I can change my wallpaper without root access.
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