– and how to achieve it ... very easily

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