As an alternative you can try loading the Gnome appearance properties screen directly at the login window. (This worked for me to change the GDM login background on my computer that is running 10.10.)
Add the gnome-appearance-properties application to the GDM login window autostart list using the following terminal commands:
sudo cp /usr/share/applications/gnome-appearance-properties.desktop .
When you logout, the appearance properties should appear in front of regular GDM login window. Any changes made will be applied to the GDM login window. When you're finished making changes close the appearance properties and log in as usual.
To stop the appearance properties from appearing again, remove the .desktop file from the login window autostart list using these terminal commands:
sudo rm gnome-appearance-properties.desktop