The com.jme3.app.state.ScreenshotAppState enables your users to take screenshots of the running game.

You activate it as follows in your simpleInitApp() method:

ScreenshotAppState state = new ScreenshotAppState();
this.stateManager.attach(state);

The default screenshot key is KeyInput.KEY_SYSRQ, alos known as "System Request / Print Screen" key. On Mac OS, you can take screenshots using Command+Shift+3 (fullscreen) or Command+Shift+4, and then press space to select a window.