ImageMagickのconvertコマンド > appendオプション

画像を結合したい時は、appendオプションを使用する

オリジナル画像：

 

縦に結合する(foo.png)

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

 横に結合する(bar.png)

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

-append

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.