Build a /map topic

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hi it might be a newbie question, i want to display a static image or picture in RVIZ(eg Aerial imagery, non-camera). the existing method is hacked the plug in for display occupancy grid map (http://wiki.ros.org/rviz/DisplayTypes/Map), and replace the occupancy values with pixel values. Now, i don't know how to get the image file directly from the /map topic. because i don't have any hardware to slam and build the map (occupancy grid). Someone can teach me how to solve this problem or any suggest?