/usr/share/gdm/dconf/10-desktop-base-settings is in desktop-base 7.0.3ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [org/gnome/desktop/background]
picture-uri='file:///usr/share/images/desktop-base/login-background.svg'
picture-options='zoom'
|