I've just done a quick comparison using 2 cubes. The bottom one is just a reference cube. No displacement at all applied. I've also used a "spots" node instead of an image, but the effect is the same. Also, I set the displacement value to 0.5, 1.0 proved excessive. (And don't forget to turn "Displacement on" in the render settings and set the minimum disp bounds to a larger value than the maximum displacement you have, in this case I set it to 1. Just in case anyone who doesn't know that looks in on this thread..) 3 images follow....