/tg/Station 13

Sticking them together in paint is the only real way afaik.

There's some kind of python tool that can generate map images, but I never got it to work. I personally just take 16 screenshots with maximum view range (it's far from 128 tiles, closer to 30; the interface lies) at pre-calculated spots so they fit together perfectly. You can reduce that to 9 and skip a lot of space pretty easily. Stitch them together in Photoshop to get the map, it takes maybe 10 minutes total with some practice.

ByondTools has a script to render a .dmm to an image.