How to pass an image (subscribed from a image publishing topic) using service?

edit

Hello People, I am trying to get image from a camera fitted on a robot and pass them using services. I will use a client to grab images from the server and later process it according to the need. It would be great if some can give me a sample template code to follow to achieve the same. But I have been receiving the following error -

error: ‘const struct sensor_msgs::Image_<std::allocator<void> >’ has no member named ‘toImageMsg’
   res.img = image_msg->toImageMsg;
                        ^
/home/dark_knight/ros_prac/catkin_ws_baxter2.1/src/robot_vision/src/vision_node.cpp: In function ‘void imageCallback(const ImageConstPtr&)’:
/home/dark_knight/ros_prac/catkin_ws_baxter2.1/src/robot_vision/src/vision_node.cpp:37:67: error: no matching function for call to ‘cv_bridge::CvImage::CvImage(std_msgs::Header, const char [5], const ImageConstPtr&)’
     image_msg = cv_bridge::CvImage(std_msgs::Header(), "bgr8", msg).toImageMsg();

Below is my code trying to do the same, I have been trying various things -

#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <opencv2/highgui/highgui.hpp>
#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>
#include "sensor_msgs/Image.h"
#include <opencv2/imgproc/imgproc.hpp>
#include "robot_vision/request_image.h"

ros::ServiceServer service;
image_transport::Subscriber sub;

sensor_msgs::ImageConstPtr image_msg;

bool get_image(robot_vision::request_image::Request  &req, robot_vision::request_image::Response &res)
{
    if(req.request_message == true)
    {
        ROS_INFO("sending back response");
        res.img = *image_msg;
    }
    else
    {
        ROS_INFO("No request received");
    }
    ros::spinOnce();
    ros::Duration(0.2).sleep();
    return true;
}
void imageCallback(const sensor_msgs::ImageConstPtr& msg)
{
  try
  {
    image_msg = msg;
   }
  catch (cv_bridge::Exception& e)
  {
    ROS_ERROR("Image data was not received");
  }
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "vision_node");
  ros::NodeHandle nh;
  //cv::namedWindow("view");
  //cv::startWindowThread();
  image_transport::ImageTransport it(nh);
  sub = it.subscribe("/cameras/head_camera/image", 1, imageCallback);
  service = nh.advertiseService("image_server", get_image);
  ros::spin();
  //cv::destroyWindow("view");
}

The above coded has been edited to work.