First image shows the material setup to use a displacement map with black producing full negative displacement, grey being neutral, and white producing full positive displacement.  The displacement value is 0.08333, but the actual range of displacement values will be from -0.08333 up to +0.08333
The idea is that you can simultaneously have both engraved and raised detail on the same polygon. ^^

Here is a test render of a simple cube demonstrating the technique:

If you only want to do one or the other, use a black background image with grey-to-white detail.  If you want to engrave, set the displacement value to a negative number;  if you want raised detail, set the displacement value to a positive number.  The displacement value should be limited to between -1 and +1. inclusive.
     Make sure in the render settings that you have ticked the use displacement maps box, and in the manual render settings, ensure that the minimum displacement bound is equal to -or greater than- your largest displacement value for any material in your scene.  If you don't,  FireFly may miss some pixels rendering because they are displaced beyond the range it was set to look.

negative displacement engraves:

positive displacement raises detail:

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5