[ros-users] Gazebo and obstacles

Hi!, I am really having problems with gazebo I cant find many info. I need to add obstacles to my world.
My obstacles are fixed, they are just rectangular boxes, the thing is that I have made a .png map and I have added this rectangles. I spawn this map using the node xml2factor_walls, but when I add this rectangles, I have an error that says that the it is not possible to connect with the spawn process.
Do you know how to spawn obstacles that are in a png map as if they were walls, so as to have the same in the gazebo world and in the map shown in the rviz.

Thanks

Nicolas