Displacement doesn't work that easy at least not in my experience.
First of all i would look at the geometry(probably a polyplane,yes?); it will displace better when you increase the number of polygons.
If that doesn't work you must go to renderingeditors=>mental ray=>approximationeditor and press create, and from there it's playing with settings until you get the desired result.

And if none of the things people say to you will work, you probably know why they're free images:lmao: