Cannot change login screen background in 18.10

Solution 1:

to change the login screen background in Ubuntu 18.10 with default Display Manager that is gdm3

we need to edit the gdm3.css file.

open the file with your favorite editor, mine is gedit. so
sudo -H gedit /etc/alternatives/gdm3.css

go to line number around 1981 and find the below content.

#lockDialogGroup {
  background: none;
  background-color: none;
  background-gradient-direction: vertical;
  background-gradient-start: #6D2169;
  background-gradient-end: #370026; }

change the content with

#lockDialogGroup {
  background: #000000 url(file:///home/pratap/Desktop/1.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center; }

here in the line number 1982 url(file://put your actual image path)

save the file, close & reboot to see the change.

Picture of Lock Screen Wallpaper

Picture of Unlock Screen Background

for 18.04 same method, only line number varies 1814,

#lockDialogGroup {
  background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
  background-repeat: repeat; }

replace the above content with

#lockDialogGroup {
  background: #000000 url(file:///home/pratap/Desktop/1.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center; }

here in the line number 1815 url(file://put your actual image path)

Solution 2:

I've created a script which automates this process of changing the login background of Gnome Display Manager (gdm).

You just need to type the script name and the image you want. You can even restore a backup of your original theme.

You can find instructions in https://github.com/thiggy01/gdm-change-login-background.