Change the login background in gnome-shell just run the script and follow the instructions A demo login-background.png file is already given
On Ubuntu 17.10 the Vanilla GNOME login screen was changed to a modified Ubuntu Version which doesn't work with this !
sudo update-alternatives --config gdm3.css
However you can use the Vanilla GNOME on 17.10 by issuing this command and selectng gnome-shell.css
That should make this work but it isn't
Even thugh the image isn't displayed in the login screen it is displayed in the lockscreen login background