Put a picture (ar_sys marker) on a urdf object

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I am using gazebo 5.0.1 with ROS Jade. I have managed to mount a camera on top of my robot and I have successfully setup the gazebo camera plugin so that I can fetch images from it. Now I want ar_sys to utilize the camera images to detect ar_sys markers that exist in front of the camera in the simulation world. The question is, how do I create the aruco markers in the world in the first place. More specifically, having these markers as .png images, how do I put them on top of a (e.g. box) urdf object?

Thanks in advance