def send_image_url(self, recipient_id, image_url, notification_type=NotificationType.regular):
        """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>
        """
        return self.send_attachment_url(recipient_id, "image", image_url, notification_type)
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>