Editing org.gnome.desktop.background in /etc/gdm3/greeter.settings doesn't change login wallpaper
 

I am trying to change my GNOME 3.4 login background using the instructions in*http://www.stderr.nl/Blog/Software/Gnome/Gnome3-Gdm3-Background.html

But that method didn't work for me. I am using Debian (Wheezy) testing. My /etc/gdm3/greeter.settings file is like this:
[org.gnome.desktop.background]picture-uri='file:///opt/wallpapers/space.jpg'
picture-options='zoom'
[org.gnome.desktop.session]session-name='gdm-fallback'
[org.gnome.login-screen]logo='/usr/share/icons/gnome/48x48/places/debian-swirl.png'
fallback-logo='/usr/share/icons/gnome/48x48/places/debian-swirl.png'
[org.gnome.power-manager]icon-policy='never'
[org.gnome.metacity]
compositing-manager=false
I have made sure that root as well as Debian-gdm user has permission to read /opt/wallpapers/space.jpg. I have also run dpkg-reconfigure gdm3 and /etc/init.d/gdm3 restart. But I don't see space.jpg as the login wallpaper. What is the issue?