Gonna add some more at Coop's post:
CTRL+F5 makes a screenshot in dosbox.
You can change the directory where the screenshot is saved in dosbox.conf if you want.
Look under header [dosbox] for
captures=capture
Whatever existing dir will work, the screenshots are automatically tagged. So you can take as many as you want.
If you don't like the CTRL+F5 hotkey it can be altered with the keymapper(CTRL+F1) in dosbox (The CTRL key is afterall usually the fire button in dosbox).
Another freeware and excellent img program is Irfanview:
http://www.tucows.com/preview/194967
You can also take screenshots with it.