You can download the GDM background changer script using
wget from a terminal:
If you only want to change the login screen colour, or apply a favourite gradient, the included
README file runs through those options.
Since I know the path to my background wallpaper it’s just a case of switching back to the terminal and using
cd to enter the downloaded script directory in a terminal. Then I run the script (with
sudo, eek) passing it the path to the image I want to use as a background
sudo ./ubuntu-gdm-set-background --image ~/Downloads/mywallpaper.jpg