I am using Debian Testing and Gnome Flashback (but it behaves the same in standard Gnome Shell).
It gives me plain white background, and I can't set my own. Also, the settings dialog for background looks funny:

Any ideas?
Edit: I have noticed I got background image back when I turned off "Nautilus controlling desktop". Of course, I no longer see desktop icons this way.
Update: I have installed Ubuntu (with Unity), keeping my /home partition. The same problem appears in Unity! I can't set desktop background.
gsettings get org.gnome.desktop.background picture-uriprint?'file:///usr/share/images/desktop-base/joy-inksplat-wallpaper_1920x1080.svg', which seems ok. Look at my edit, the problem exists only when I want to have icons on the desktop. BTW, should this dialog really look like this?