If you have the Saitek Yoke & Throttle Quadrant you can use Button 2 (the red button on the top right yoke handle). Or, if you don't use ATC , Button 1 is very accessible the left index finger (red button on the back side of the left yoke handle).
I don't have FSX installed right now, but I believe you can go to the:
-> FSX Main Page -> Settings -> Controls -> Assignments (?) --> SCROLL & CHOOSE -> (Saitek ?) Yoke & Throttle -->
A list of all default assignments should appear.
* Click on one of the assignments ONCE.
** This will ensure that you will scroll through the assignments and NOT continue scrolling through the controlling devices.
SCROLL down to (I believe it's...) SCREENSHOT. It will list the default key stroke as "V" right next to it (in any case) and the JOYSTICK ASSIGNMENT (?) column should be blank. Anyway,
DOUBLE CLICK the row with the SCREENSHOT & "V"
--> a window will open asking if you want assign a button to a task -->
PRESS the red button (or whichever other button you wish to assign the task of taking the screenshot). When you press the button, "Button 2" (or whatever other button you wish to assign) should appear in the box in the window.
I believe it will ask if you're sure you want to do this, CLICK YES.
The window should disappear and the row should now read "SCREENSHOT" (Task Column) "V" (Key Stroke Column) and Button 2 (Joystick Assignment Column).
Then click ok at the bottom.
This should put you back to the MAIN SETTINGS PAGE.
Usually I double check to make sure that the assignment was saved by simply going back to Controls -> CHOOSE Yoke & Throttle -> CLICK ONCE on an assignment -> SCROLL down to SCREENSHOT and ensure the Joystick Assignment still reads BUTTON 2.
Exit out of the settings page
-> load a flight
-> press Button 2
-> ESC
-> End Flight
-> CLICK on (can't remember the folder) that has the Pilot Awards, Records & Pictures (I'm pretty it's 3 down from "Create a Flight" on the Main Page
-> CLICK MANAGE PICTURES, and your new picture should be there ready for editing (or whatever).
I apologize for not having all the exact tab names, but hopefully this is close enough and clear enough so that you can figure it out. Anybody feel free to copy and paste, make any clarifications and corrections for any other interested parties.
You can also use FSUIPC and SPAD.neXt to assign controller buttons, joystick assignments and bunch of other things. After 14 years experience, I just figured out how powerful and useful FSUIPC is about a week ago, so don't feel bad if you've been at this for a while. FS has A LOT of sub-disciplines and sub-interests. OldAirMail introduced me to SPAD.neXt which I'm just starting to try out now. Then that lead me Arduino which lets you build your own fs controller devices and such - so there's plenty to learn for everyone!
Best wishes to all!
Dave