Sets a custom image for the game. `image_path` should refer to
        an image file on disk

def set_image(self, user, image_path):
        """Sets a custom image for the game. `image_path` should refer to
        an image file on disk"""
        _, ext = os.path.splitext(image_path)
        shutil.copy(image_path, self._custom_image_path(user, ext))