Depth to mud

i see maps where the vehicle sinks in the mud. How do i add it to my map?

 

Copy and paste on sure, but it would only work in certain areas. I created a custom spot on derby arenas, sort of like a straight line mud drag, i just need the add the depth there. But im not sure where to draw the gray line in mx fireworks in order to create the depth, is it just guess and check?

 

got that done, how to i actually make the depth, do i just color a gray line into a white png whereever the mud is?

 

i took the depth ter png and put it over the mud height map. colored it perfectly and nothing. im using macromedia fireworks 2004 so its pretty outdated. Do i need to reimport the heightmap while in the editor>