you can use a displace modifier

https://www.blender.org/manual/modeling/modifiers/deform/displace.html

[![enter image description here][1]][1]


[![enter image description here][2]][2]



[![enter image description here][3]][3]


[![enter image description here][4]][4]


  [1]: https://i.sstatic.net/K4sAf.jpg
  [2]: https://i.sstatic.net/BYK2K.png
  [3]: https://i.sstatic.net/F9XXN.png

And then Use a Cast modifier on another plane to make a flat version
that you can extrude

  [4]: https://i.sstatic.net/HrGkW.png
 

maybe throw in a simplify modifier to lower the vertices even more before extruding 


But to get a good detail one you need an insane amount of faces in the start.
Cut up the original image to only use one shape at a time
and you will get better resolution with less faces.

Maybe there is a way to mask the texture and extrude the mask?