Official ImageMagick logo from Wikmedia Commons (GPL 2).
If you make websites, you've probably worked with Imagemagick and know it's a pain in the butt to get working (especially for Mac local development)—and if you want to use an awesome CMS like Omeka (super for digital archives, collections, museums) to display still images, you'll need Imagemagick. Used to be you'd need to lose some time finagling (possibly setting up Homebrew) to get ImageMagick to work, but if you're already using MAMP, it might be worth your while to upgrade to MAMP Pro 3 and make things easier on yourself.
These instructions assume knowledge of installing an Omeka site locally using MAMP.
Look under the PHP tab in the MAMP Pro 3 settings window to locate the version of PHP MAMP is using.
Type the ImageMagick path at the bottom of the page in the Omeka dashboard settings.
Note that another way to make Imagemagick finagling simpler is to use Vagrant (if you've ever worked with a virtual machine, for example to run Windows on a Mac, it's like developing in a virtual machine and being able to easily boot up a machine with a new copy of the CMS of your choice configured how you want, easily move that dev site to a public site, and not have any of this mess with the settings on your host computer/laptop the way you might need to do when using MAMP).
In particular, the awesome digital humanities devs at Scholars' Lab share Vagrant+Chef code and documentation for Omeka sites at this GitHub repo (haven't actually used it myself yet, but planning to very soon). Working with Vagrant takes some learning, though, so if you don't have the time/desire to add a useful webdev skill to your toolkit just now, the MAMP Pro 3 route might be better for you.
I started using MAMP Pro 3 to make local Omeka dev easier after user niek noted its awesomeness on the Omeka Dev listserv (hat tip!).