Easily Combine Images On a Mac

1. Install imagemagick (from MacPorts or Homebrew): port install imagemagick or brew install imagemagick
2. convert +append source_image1.jpg source_image2.jpg source_image2.jpg combined.jpg

Voilà!

You can combine multiple images not just three. You can also do convert +append * .jpg combined.jpg

Combine comes with a long manual that you can find here or by typing man convert

convert(1)                                                           General Commands Manual                                                          convert(1)



NAME
       convert - convert between image formats as well as resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.

SYNOPSIS
       convert [input-option] input-file [output-option] output-file

OVERVIEW
       The convert program is a member of the ImageMagick(1) suite of tools.  Use it to convert between image formats as well as resize an image, blur, crop,
       despeckle, dither, draw on, flip, join, re-sample, and much more.

       For more information about the convert command, point your browser to file:///opt/local/share/doc/ImageMagick-6/www/convert.html or
       http://imagemagick.org/script/convert.php.
...

This tool will also work on other systems where you can install combine such as FreeBSD or Linux.