Suppose you wish to change the background for GDM (GNOME Display Manager) to /usr/share/backgrounds/zim2687.jpg.
Here is how to do it:
# xhost +SI:localuser:gdm # sudo -u gdm gsettings set org.gnome.desktop.background picture-uri 'file:////usr/share/backgrounds/zim2687.jpg' # xhost -SI:localuser:gdm
Log out and you should see the new background when GDM restarts.
