This packages provides publisher and subscriber plugins for the ROS image_transport package.
If you are looking for a way to convert between sensor_msgs::Image and sensor_msgs::CompressedImage messages using the ImageZero compression algorithm, you should look at the imagezero_ros package.
If you are using image_transport, you do not need to directly use either of these classes. As long as this package is in your ROS package path, any image_transport::Publishers will automatically publish an "imagezero" topic, and any image_transport::Subscribers can be made to subscribe to it by setting their ~image_transport parameter to "imagezero".