I assume the normal render window looks fine, correct?
Can you try something a bit easier?
- Run: gazebo worlds/camera.world
- When the GUI appears, press CTRL-t to open the topic visualizer.
- Select one of the camera image topics, should be /gazebo/defualt/camera/link/camera/image
- Does the image look correct? If so, try to fiddle with the environment (add objects, move the camera, etc) to see where it breaks.