ImageMagickのconvertコマンド > appendオプション


$ convert -append baz.png baz.png foo.png
 $ convert +append baz.png baz.png bar.png

ImageMagick逆引きコマンドリファレンス   [本]


Join current images vertically or horizontally.
This option creates a single longer image image, by joining all the current images in sequence top-to-bottom. Use +append to stack images left-to-right.
If they are not of the same width, narrower images are padded with the current -background color setting, and their position relative to each other can be controled by the current -gravity setting.