/usr/share/slim/themes/altusmetrum/background.png is in altos 1.6-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/slim/themes/altusmetrum/../../../altos/themes/background.png
/usr/share/slim/themes/altusmetrum/../../../altos/themes/background.png