Bug#637343: Seems to be a problem with .svg files and/or location
The 'spacefun-wallpaper.svg' was the only background
available, ,and then only because it is default. Every other
background that is in the list resides in '/usr/share/backgrounds/gnome'.
When I tried to add any of the .svg backgrounds
(i.e. 'moreblue-orbit-wallpaper.svg') the file chooser showed a '?' symbol,
whereas selecting one of the .png files gives a thumbnail view of the
background. After selecting an .svg file to add, the resulting choice
showed a solid green background in the list of choices (no background).
I realize this is a probably a corner case, after all the other machine
I administer is able to display the background just fine. After upgrading
this system to the same level as the working one I expected this to be fixed
(the upgrade included some gnome dependencies and inkscape), so
I guess I'll keep drilling and GDM, s and ltracing. Also I'll reinstall
'gnome-control-center' and have a look at the 'gnome-appearance-properties'