Hi,
you need to craete a displacement shader. Then connect the outAlpha of the texture you want assign as displace to the input of the displacmeent node.
Adjust carefully the outAlpha of your texture because the displace will offset the object. To compense, set the half negative in alphaOffset (i.e. alpha gain 0.5, alpha offset -0.25)
Hope this helps...
Emmanuel
Emmanuel Campin
Lighting & shading TD
www.pixho.com