Stumbled across a method of changing the gdm login wallpaper, it's not that complex, but I didn't think tools were often used to control settings in UNIX operating systems so this is worth a mention.

sudo -u gdm gconftool-2 -t string -s /desktop/gnome/background/picture_filename /usr/share/backgrounds/Butterfly.jpg

If you're interested to see what settings can be tweaked

sudo -u gdm gconftool-2 -R /

I'm not overly impressed with this method myself, perhaps building this list from a text file would be more in tune with the UNIX way. Of course it's not that far out, but it's definitely not the method that I'd expect.