I dont know if there is a special way to do it in RPG Maker but...
if you press this button
you can copy the screen and then you go into paint and hit paste
some keyboards are different but usually it's in the same place...
So basicly you go to a part of the game you want to take a screen shot of... then you hit the button, then you copy it into paint and then you edit out the unwanted "extras"
SO THERE! NOW EVERY ONE KNOWS HOW TO TAKE A SCREEN SHOT