Appending images vertically in ImageMagick
For vertical stacking (top to bottom):
convert -append 1.jpg 2.jpg out.jpg
For horizontal stacking (left to right):
convert +append 1.jpg 2.jpg out.jpg
Use -append to stack images top-to-bottom.
Use +append to stack images left-to-right.
Source: http://imagemagick.org/script/command-line-options.php#append