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