Hector_quadrotor Indoor slam : no image recieved

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I am running the indoor slam tutorial for Hector quadrotor, in Rvis the two main topic related to camera are on the topic list ( /front_cam/camera/camera_info, /front_cam/camera/image. i cant read any image from the second topic /front_cam/camera/image(by subscribing ). in Rvis there is yellow sign on the Camera (warn) and : camera info, visibility and topic are all OK but Image : Ni image received. any suggestions , plaese?

I am running ROS kinetic , Gazebo 7.12 on Ubuntu 16.04 on Vbox.