How to change login wallpaper in Ubuntu 18.04

Open terminal and go to the following path:

sudo gedit /usr/share/gnome-shell/theme/gdm3.css

Now look for #lockDialogGroup:

You should see something like:

#lockDialogGroup {
  background: #2c001e url(resource:///org/gnome/shell/theme/<some-file>.png);
  background-repeat: repeat; }

Change it to:

#lockDialogGroup {
  background: #2c001e url(file:///home/jsbisht/Pictures/wallpapers/On_top_of_the_Rubihorn_by_Matthias_Niess.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center; }

Reboot and you should see the login wallpaper applied.

NOTE

You can copy default ubuntu wallpapers to your Pictures directory as follows:

mkdir ~/Pictures/wallpapers
cp -r * ~/Pictures/wallpapers/