/usr/share/themes/Blackbird/xfwm4/bottom-left-inactive.xpm is in blackbird-gtk-theme 0.4+20160704~ab4a30e-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/themes/Blackbird/xfwm4/bottom-left-active.xpm