/usr/share/wallpapers/kde-default.png is in kde-wallpapers-default 4:4.8.2-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/wallpapers/Ariya/contents/images/1920x1200.png
/usr/share/wallpapers/Ariya/contents/images/1920x1200.png