how to subscribe to usb_cam/image_raw to process the image from camera?

edit

what is missing in the below code that is used for subscribing to the topic /usb_cam/image_raw.

void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "main");
  ros::NodeHandle n;

  std::cout<<"Subscribing starts!"<<std::endl;
  ros::Subscriber sub = n.subscribe("usb_cam/image_raw", 2, chatterCallback);

  ros::spin();
  return 0;
}

how to get the image from the camera so that i need to process this image. Is there any files missing in the header.

~/usb_cam/launch$ rostopic info /usb_cam/image_raw
Type: std_msgs/String

Publishers: None

Subscribers: 
 * /listener (http://ubuntu:42711/)
 * /main (http://ubuntu:35605/)