you can use a displace modifier
https://www.blender.org/manual/modeling/modifiers/deform/displace.html
And then Use a Cast modifier on another plane to make a flat version that you can extrude
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. MaybeCut 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?
cutting out the shapes and doing one at a time would help too