join multiple png images
ImageMagick can do this:

convert 1.png 2.png -append result.png


Alignment options can be (optionally) specified using the -gravity switch,


-gravity East align the right edges
-gravity West align the left edges
-gravity center align the centers


Note: using +append instead of -append will stitch the images together horizontally, i.e. side-by-side rather than one above the other.