Temporary SOLUTION:
I've found another workaround. Unfortunately for us this doesn't seems
to be a bug, at least I guess there's no intention from gnome developers
to solve it soon:
A upstream developer said: You shouldn't start GNOME programmes with
elevated privileges, it's not supported. In
I found THE solution!
(thanks to Ali for his information in comment #10)
I had the same problem.
It appears to be that Ubuntu seeks for XML entrys in ALL the files in
/usr/share/gnome-backgroud-properties directory, not only the xml files
(you can even have a file named my-wallpapers.ogg but
