Return the image of the given game type from the screen.
        Return None if no game is found.

def _game_image_from_screen(self, game_type):
        """Return the image of the given game type from the screen.
        Return None if no game is found.
        """
        # screen
        screen_img = self._screen_shot()
        # game image
        game_rect = self._game_finders[game_type].locate_in(screen_img)
        if game_rect is None:
            return
        t, l, b, r = game_rect
        game_img = screen_img[t:b, l:r]
        return game_img