There is currently no way to take screenshots in-game without using third-party software such as FRAPS or using the Steam Overlay. Third-party software is a drag on performance and the Steam Overlay does not work for me (SHIFT-TAB does nothing and F12 only brings up a command window; I have Steam Friendly UI enabled and the Steam In-Game enabled in my Steam settings). PrintScrn only produces a pure black image for me and requires ALT-TABing.
Given that the game includes a "Splendid Camera" with many pan/zoom functions it might as well have a native "Take Picture" function like most other games do to allow players to avoid all the extra hassle of setting up screenshot sofware.