DF had a built-in screenshot function meant for use in pure DOS mode. You start the game with "dark.exe -shots", and then press the Print Screen key ingame. They appear as PCX files in the same directory as dark.exe.
At least, that's what I remember. I didn't test it just now.
But you'd be better off having a Mac user take them anyway, with the higher resolution and longer viewing distance.
As always, you should probably go to DF-21.net if you want more info. I don't know why they even have this forum, if nobody who knows DF goes here.