/usr/share/wallpapers/kde-default.png is in kde-wallpapers-default 4:4.13.0-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/wallpapers/Elarun/contents/images/2560x1600.png
/usr/share/wallpapers/Elarun/contents/images/2560x1600.png