Exec() has been disabled

[hyperion]

Php.ini allows the server administrator to disable certain functions. If the server administrator has disabled exec() you will not be able to use Image Magick.

You may replace exec() with passthru() if it has not been disabled as well.  Otherwise, you must use GD.

[Just]

Forgive the ignorance, but what do you mean by use GD?

[Joachim Müller]

http://coppermine.sourceforge.net/faq.php#WhatIsGD
http://coppermine-gallery.net/demo/cpg13x/docs/index.htm#config_picture_thumb_advanced

Method for resizing images
If you are using GD 1.x and the colors of your thumbnails or intermediate image are wrong then switch to GD 2.x