How to create wall in gazebo from a grayscale image?

edit

Hello, as explained in the question, I'm trying to create walls in Gazbo from a grayscale image that I have. I used he Image tags in SDF :

<sdf version="1.4">
<model name="institut">
<link name="link">
<visual name="instiut">
<geometry>
<image>
<uri>model://institut/materials/map_gazebo.png</uri>
<height>2.0</height>
<scale>10.0</scale>
<threshold>250</threshold>
<granularity>1</granularity>
</image>
</geometry>
</visual>

<collision name ="coll">
<geometry>
<image>
<uri>model://institut/materials/map_gazebo.png</uri>
<height>2.0</height>
<scale>10.0</scale>
<threshold>250</threshold>
<granularity>1</granularity>
</image>
</geometry>
</collision>

</link>
</model>
</sdf>

There is no error or warning when inserting the model in Gazebo, but nothing appears. (The model name is listed in the inserted models list)

Can anyone spot an error or had anyone some experience with this? Thanks.