Ubuntu – How to change the login screen in Ubuntu 16.04+

You said:

The "custom" background shows for a split second and then fades to the Ubuntu one.

The greeter by default load the selected user background if it is available. A fade transition used to change between backgrounds of users including greeter default (for guest user). I think that the question should: How to stop lightdm greeter from loading the user background?

Be aware of two distinct greeters, default is unity-greeter. I noticed that some answer here in AU, don't mention this and mix up between them.

Using lightdm-gtk-greeter

GUI tool

1. Install the settings tool

   sudo apt install lightdm-gtk-greeter-settings
   

2. Run it

   pkexec lightdm-gtk-greeter-settings
   

3. In Appearance tab: Select the image

4. On same tab: Unckeck User user wallpaper if available

5. Save & Close

   

CLI tools

1. Open lightdm greeter settings file

   sudo nano /etc/lightdm/lightdm-gtk-greeter.conf
   

2. Change it this way

   [greeter]
   background = /usr/share/backgrounds/Spring_by_Peter_Apas.jpg
   user-background = false
   

3. Ctrl+o to save it then Ctrl+x to exit.

Using unity-greeter (default)

Same answer by Serg, I have tested it in a fresh VBox Ubuntu 16.04.

• Change the background of the login screen

  $ sudo nano /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override
  
  [com.canonical.unity-greeter]
  draw-user-backgrounds=false
  background='/usr/share/backgrounds/Spring_by_Peter_Apas.jpg'
  
  $ sudo glib-compile-schemas /usr/share/glib-2.0/schemas
  $ sudo service lightdm restart
  

• Some debug hints:

  1. Purge lightdm-gtk-greeter settings

     sudo apt purge lightdm-gtk-greeter lightdm-gtk-greeter-settings
     

     Or you may want just disabling it temporary

     sudo mv /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf.disabled
     

     To enable it back

     sudo mv /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf.disabled /usr/share/lightdm/lightdm.conf.d/60-lightdm-gtk-greeter.conf
     

     Restarting the lightdm is needed

     sudo systemctl restart lightdm
     

  2. Reinstall unity-greeter to remove any change in /usr/share/glib-2.0/schemas/com.canonical.unity-greeter.gschema.xml

     sudo apt install --reinstall unity-greeter
     

  3. Check if there any other dconf override file that take priority

     grep -rn -e "com.canonical.unity-greeter" -e "background=" -e "draw-user-backgrounds=" /usr/share/glib-2.0/schemas/