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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def custom_image(self, user):
 """Returns the path to the custom image set for this game, or None if no image is set"""

        for ext in self.valid_custom_image_extensions():
            image_location = self._custom_image_path(user, ext)
            if os.path.isfile(image_location):
                return image_location
        return None