Download the photo associated with a Savings Goal.

def get_image(self, filename: str=None) -> None:
        """Download the photo associated with a Savings Goal."""
        if filename is None:
            filename = "{0}.png".format(self.name)

        endpoint = "/account/{0}/savings-goals/{1}/photo".format(
            self._account_uid,
            self.uid
        )

        response = get(
            _url(endpoint, self._sandbox),
            headers=self._auth_headers
        )
        response.raise_for_status()

        base64_image = response.json()['base64EncodedPhoto']
        with open(filename, 'wb') as file:
            file.write(b64decode(base64_image))