/usr/share/unity/themes/bfb_bg_prelight.png is in unity-asset-pool 0.8.24daily13.06.10-0ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/unity/themes/bfb_bg_normal.png
/usr/share/unity/themes/bfb_bg_normal.png