black to transparent

convert in place, all black to transparent

mogrify -transparent black *

mirroring

horizontal mirroring

mogrify -flop *.png

vertical mirroring

mogrify -flip *.png

hacky texture atlas

append to image horizontally (left to right)

convert +append *.png out_01.png

and vertically (top to bottom)

convert -append out_01.png out_02.png out_03.png tilemap.png

this is useful for texture atlas, last argument is output