<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update_image(self, image_id, user_name, desc=None):
 """Create or update an Image in Image Registry."""

        desc = desc if desc else ''
        data = {"username": user_name,
                "description": desc}

        return self._post('/images/%s' % image_id, data)