Just to make sure that you're doing it correctly (it should work, unless your print screen button is broken
):
1. Get to the place in the game that you want to take a snapshot
2. Press the "Print Screen" button on your keyboard
3. Either exit out of the game or Alt-Tab out of the game
4. Open up your image editing software (Microsoft Paint will work if you don't have anything else)
5. Either create a new image and select the paste option or some software has the option "Paste as New Image"
I know this works, I've done it plenty of times.