First of all, open up the console in either single player or multiplayer by hitting shift and ~.
Next, bind a key you're not using as a control to the screenshot command with the following command:
bind n screenshot (I've used n in this example)
This will update your config file(s) in your JKII\GameData\base directory. Hit ~ again to close the console.
When you take a screenshot by pressing the n key, it'll be saved in a directory called JKII\GameData\base\screenshots.