Just in case others were curious about how to change your GDM background wall paper, I did it a simple way. I searched for awhile and found people using gconftool-2 to do this when all I did was the following...
Thats it.
Code:
# cd /usr/local/share/pixmaps/backgrounds/gnome
# mv background-default.jpg background-default1.jpg
# cp /home/user/Images/wallpaper/newWallpaper.jpg background-default.jpg