/usr/bin/montage-im6 is in imagemagick 8:6.8.9.9-7ubuntu5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../lib/x86_64-linux-gnu/ImageMagick-6.8.9/bin-Q16/montage
/usr/bin/../lib/x86_64-linux-gnu/ImageMagick-6.8.9/bin-Q16/montage