Post 67: Set background image in ubuntu 18.04

Type in your terminal:

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

Then search for "lockDialogGroup". Replace the CSS query with the following:

#lockDialogGroup {
background: #2c001e url(file:///[fileLocation/filename.png]);
background-repeat: no-repeat;
background-size: cover;
background-position: center;
}