Some of the features of the CodeIgniter image manipulation class require GD2, however, other features require ImageMagick. If you're using MAMP on MAC, then the chances are that you won't have it installed by default. Cactuslab have produced an installer that does the job for you.
Download the installer. At the time of writing this, the latest version is ImageMagick 6.8.6-3 for Mac OS X 10.5 - 10.8.
Run the installer, and if all goes well, you should now have ImageMagick installed. You'll need to set the $config['library_path'] value to /opt/ImageMagick/bin, as follows:
'library_path' => '/opt/ImageMagick/bin'
How it works...
The installer takes care of everything for you: it's voodoo!