This program is member of the ImageMagick(1) suite of tools.
In my case, its already installed in my bash term, so i didn't install anything.
If you want more information about this command you can go to ImageMagick site or just type man convert in bash
How to use it:
- convert img1.jpg img2.jpg [img3.jpg ... imgN] +append new_img.jpg
- convert img1.jpg img2.jpg [img3.jpg ... imgN] -append new_img.jpg