I use the ImageMagick built for Mac OS X that is part of the Graphics Libs and available for free at this web site
http://www.kyngchaos.com/software/unixport/graphicslibs
The site currently offers for Mac OS X ImageMagick 6.2.8-3 in two flavors: one supports images with no more than 8 bits per color channel, the other support images with up to 16 bits per color channel.