it's pretty easy.
1) Create a new blinn, lambert whatever.
2) Create a new 2D file texture node.
3) Assign your jpg of the "bump" to the file node.
4) Drag the file node while holding the middle mouse button, onto your blinn, and you should then get a menu where you can choose displacement.
5) Assign texture to object.
BTW, my earlier post about the 3d texture node was false.
These things help to control it:
Increasing tesselation of object and also playing with the Alpha Gain > under Color Balance of the file node of the "bump" image.
You won't see the displacement until render time.
Here is also a tut explaining the whole process: