If you press alt+s in game it will take a screen shot of what you see at the time it's taken. You can then find these screen shots in your Aces High 2 Folder.
If you want to get more technical with screen shots, then you can record your flight with alt+r and then stop recording again with alt+r. Then open the film in the Aces High Film Viewer (also located in the Aces High Folder), play the film to the part where you want a screen shot from, pause it, and using the features in the film viewer, set up the camera position anywhere you want it (inside the plane, out side, anywhere). Once you have the camera positioned, click the "View Full Screen" button, press "Print Screen" on your keyboard, and paste it into and image editing software (MS paints is easiest). Just remeber you move the mouse out of the screen when in full screen view.