A OS X package of ImageMagick is available at
http://www.entropy.ch/software/macosx/welcome.html#imagemagick
But is doesn't 'just work'...
...you should also go into the terminal and enter the following command;

echo "export PATH="/usr/local/bin:$PATH"" >>~/.bash_profile

## this lets the GLI find imageMagick by adding the 'export[...]' line
to your .bash_profile ##

Regards,

Stephen