How to merge images in command line?

You can also use GraphicsMagick, a lighter and faster fork of ImageMagick:

gm convert image1.png image2.png -append combined.png

A simple time comparison of merging 12 images:

time convert image{1..12}.jpg -append test.jpg

real    0m3.178s
user    0m3.850s
sys     0m0.376s

time gm convert image{1..12}.jpg -append test.jpg

real    0m1.912s
user    0m2.198s
sys     0m0.766s

GraphicsMagick is almost twice as fast as ImageMagick.

You can also try ImageMagic which is great for creating CSS sprites. Some tutorial about it here.

Example (vertical sprite):

convert image1.png image2.png image3.png -append result/result-sprite.png

Example (horizontal sprite):

convert image1.png image2.png image3.png +append result/result-sprite.png