f1 quote
The displacement height is the alpha value of the texture map by default. If you turn on Alpha is Luminance for the texture, the displacement height results are based on the intensity of the pixels in the texture map.
endquote...

then you can tweak it with the contrast controls on the texture/file node.

if it doesnt gets the shape of the pic youre using slowly up the samples of the texture (the more samples the more polys)