Running the ROS node

I have some problem starting the Mono node from the original SLAM2.
when I do 

> rosrun ORB_SLAM2 Mono <path_to_vocabulary_file> <path_to_yaml_file>

it starts and loads the vocabulary but then the frame window remains empty and says waiting for the images. So whats the method to start Monocular node from topic **/camera/image_raw?**  