imagemagick doesn't delete tmp files

As of January 2013, there is no official way to delete temporary files, so imagemagic leaves you to do it yourself. I also use a cron job that runs every 20 minutes since the temporary files are 10+ GB in size.

It means your ImageMagick installation is NOT functioning properly!

The fact that it leaves magick-* files in tmp says that ImageMagick process died and that's why it didn't delete those files. You should configure memory limits etc. Refer http://www.imagemagick.org/script/resources.php

I've had this problem for a period of time, my code issue was that I didn't use the destroy function:

$im->destroy();

When calling the destroy function it will delete the temp files that are made under the /tmp directory. This is for PHP and maybe it will help someone.