/usr/share/xfce4/backdrops/xubuntu-karmic-gdm.png is in xubuntu-wallpapers 12.04.11.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/xfce4/backdrops/xubuntu-karmic.png
/usr/share/xfce4/backdrops/xubuntu-karmic.png