As mentioned before, PHP is misconfigured on OSX and you don't get their customized sourcecode in order to recompile it with the options you need. As such you can't run the GD plugin which is what most OSS programs use for image processing, and is generally the most recommended way of doing it. For Drupal's ImageCache / ImageAPI module(s) what you can do instead is install the ImageMagick package and then just tell Drupal to use the ImageAPI-ImageMagick module instead of the GD one. I've already got ImageMagick installed on my McBook for Rails use so it was only a few seconds to get it going for Drupal too.