/usr/share/xfce4/backdrops/greybird-wall-1920x1200.png is in shimmer-wallpapers 1.3.0-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/xfce4/backdrops/../../wallpapers/greybird-wall-1920x1200.png