Change GDM wallpaper in Linux Mint 12

/usr/share/backgrounds/linuxmint $ ls -l

total 0

lrwxrwxrwx 1 root root 47 2012-01-29 23:48 default_background.jpg -> ~/Pictures/wallpaper/lion_space1.jpg

lrwxrwxrwx 1 root root 34 2012-01-18 15:12 default_background.jpg.orig -> ../linuxmint-lisa/gelsan_green.png