Activate ScreenCapture module on Modules tab in Orbiter launchpad. When in simulation press Alt-F12 to make a screenshot. If you want to save only a certain region of a screen, press Alt-F11 and select a region by pressing the left mouse button and dragging the cursor. Once you release the button the image will be saved. To cancel selection press the right mouse button.
Images are saved to the "(screenshots)" folder in Orbiter root directory.
There is the ScreenCapture.cfg file in Config directory, where you can specify the output image format (jpg or bmp) and the quality of jpg images.