Background image code should use GnomeBG
from libgnome-desktop (https://developer.gnome.org/gnome-desktop3/3.26/gnome-desktop3-gnome-bg.html)
Edited by Guido Gunther
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from libgnome-desktop (https://developer.gnome.org/gnome-desktop3/3.26/gnome-desktop3-gnome-bg.html)