/usr/share/xfce4/backdrops/nsmail.png is in ubuntustudio-wallpapers 0.48.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/xfce4/backdrops/../../backgrounds/ubuntustudio/nsmail.png
/usr/share/xfce4/backdrops/../../backgrounds/ubuntustudio/nsmail.png