This is a script which automates the process of setting an image in the Gnome Display Manager background of Ubuntu in order to change its famous aubergine theme.
This script was tested in Ubuntu 18.04, 18.10, 19.04 and 19,10 only and I'm not responsible for any problem that it may cause. THIS SCRIPT WON'T WORK WITH THE NEW UBUNTU 20.04. If you want to change Ubuntu 20.04 gdm backgroud image, you have to use another script avalible at https://github.com/thiggy01/ubuntu-20.04-change-gdm-background.
First of all, You need to have yad package installed to be able to see the
graphical user interface. On Ubuntu, you can install it with sudo apt install yad.
Download the script with the following command:
curl -L -O github.com/thiggy01/ubuntu-change-gdm-background/raw/master/ubuntu-change-gdm-background
Set it as executable with chmod +x ubuntu-change-gdm-background command and
run it with root privileges such as sudo ./ubuntu-change-gdm-background.
If you see a message login image sucessfully changed, then, when you restart gdm
servive or reboot your computer, your login background should be covered with the
image you selected.
You can restore your original gdm theme any time with sudo ./ubuntu-change-gdm-background --restore.
This script will save the pictures in /usr/share/gdm/saved-pictures folder so you can move your original image file without losing the gdm login image setting.