code: function sendImageMessage(recipientId, imageURL, token) {
        imageMessageData = {
          attachment:{
            type:"image",
            payload:{
              url:imageURL
            }
          }
        }
        request({
          url: FB_MSG_URL,
          qs: {access_token:token},
          method: 'POST',
          json: {
            recipient: {id:recipientId},
            message: imageMessageData,
          }
        }, function(error, response, body) {
          if (error) {
            node.log('Error sending iamge message: ', error);
          } else if (response.body.error) {
            node.log('Error: ', response.body.error);
          }
        });
      }
docs: Send out image message (of msgType: image)