Here is a working example of montageImage. The example does a color analysis of a photo. To make sure it works properly, you will need to make sure:
You have a valid "test.png".
Imagick::clone has been deprecated as of imagick 3.1.0 in favour of using the clone keyword. So if you have imagick 3.1.0, you will need to change lines 13 and 17.
$bright = $average->clone();
to
$bright = clone $average;
and
$dark = $average->clone();
to
$dark = clone $average;
Good luck.