You've all seen those grungy looking portraits ... and you may have wondered how long the road ahead to achieve it, is.

Let me tell you: It's a very short trip.

Open your image (Pro tip: it doesn't have to be a portrait :D ) in Photoshop

Now make two duplicates of the background layer. Press CMD (mac) or CTRL (pc) + J twice, and your layer palette looks like this:

3 exact copies of the same layer.
Now select the layer in the middle. We need to desaturate that completely. You could either go to Image -> Adjustments -> Desaturate, og you could simply press CMD (mac) or CTRL (PC) + SHIFT + U

And now you have this

With the middle layer now desaturated, or black & white. 
Now select the top layer, og go set the layer blend mode to OVERLAY

And you're done ... and have now achieved that popular grungy look

Merry Christmas