The lock screen shield is the screen that quickly slides down when the system is locked. The background of the lock screen shield is controlled by the org.gnome.desktop.screensaver.picture-uri GSettings key. Since GDM uses its own dconf profile, you can set the default background by changing the settings in that profile.
Create the gdm profile which contains the following lines:
user-db:user system-db:gdm file-db:/usr/share/gdm/greeter-dconf-defaults
gdm is the name of a dconf database.
Create a gdm database for machine-wide settings in /etc/dconf/db/gdm.d/01-screensaver:
Replace /opt/corp/background.jpg with the path to the image file you want to use as the lock screen background.
Supported formats are PNG, JPG, JPEG, and TGA. The image will be scaled if necessary to fit the screen.
Update the system databases:
# dconf update
You must log out before the system-wide settings take effect.
Next time you lock the screen, the new lock screen shield will show in the background. In the foreground, time, date and the current day of the week will be displayed.
Make sure that you have run the dconf update command to update the system databases.
In case the background does not update, try restarting GDM.
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