http://docs.ros.org/api/image_transport/html/

Code API
When transporting images, you should use image_transport’s classes as drop-in replacements for ros::Publisher and ros::Subscriber.

image_transport::ImageTransport - use this to create a Publisher or Subscriber
image_transport::Publisher - manage advertisements for an image topic, using all available transport options
image_transport::Subscriber - manage an Image subscription callback using a particular transport
Camera drivers publish a “camera_info” sibling topic containing important metadata on how to interpret an image for vision applications. image_transport included helper classes to publish (image, info) message pairs and re-synchronize them on the client side:

image_transport::CameraPublisher - manage advertisements for camera images
image_transport::CameraSubscriber - manage a single subscription callback to synchronized image (using any transport) and CameraInfo topics
For other synchronization or filtering needs, see the low-level filter class:

image_transport::SubscriberFilter - a wrapper for image_transport::Subscriber compatible with message_filters