i've triad f12 and print key so far but it doesn't seem to work

Are you using Steam? If yes, just add Miasmata to your games list (how to) and you can screenshot away using F12. If you're not a Steam user, it's a bit more complicated:
First, get Fraps. Freeware version will do. Now, launch it and browse trough the settings - especially take note of Overlay key and screenshot key. With Fraps launched, launch Miasmata and press the screen overlay key (a yellow number should appear in one of the corners of your screen.) When that happens, press the key you have set to capture a screenshot. You may turn the overlay off after that. Now, the screenshot should be saved somewhere in the folder of Fraps.

thx.

yeah, i own the gog version, so i try it with fraps!

Doesn't matter, you can add non-steam games to Steam and then use its screenshot functionality to create screenshots.

sorry, this solution doesn't work for me. i've just added miasmata to steam and tried to make screenshots.
steam didn't make any. i'll have to stay at fraps and convert the screenshots to a proper file format like .png later on.