A script which automates the process of changing the Gnome Display Manager login screen background image of Gnome Display Manager version 3.
Download it with the following command:
curl -L -O github.com/thiggy01/gdm-change-login-background/raw/master/gdm-change-login-background
Give it execution permition with chmod +x gdm-change-login-background command and run it with root privileges such
as sudo ./gdm-change-login-background path/to/image/file.
If you see a message login background image sucessfully changed, then, when you restart gdm or reboot your computer,
your gdm login background should be covered with the image you selected.
You can restore your original gdm theme any time with sudo ./gdm-change-login-background --restore.