code: def send_image_url(self, recipient_id, image_url, notification_type=NotificationType.regular):
          
          return self.send_attachment_url(recipient_id, "image", image_url, notification_type)
docs: Send an image to specified recipient using URL.
          Image must be PNG or JPEG or GIF (more might be supported).
          https://developers.facebook.com/docs/messenger-platform/send-api-reference/image-attachment
          Input:
              recipient_id: recipient id to send to
              image_url: url of image to be sent
          Output:
              Response from API as <dict>