If you want to crop your image based on person's face, then there is no need in separating the background from the foreground. Nowadays you will easily find ready implementations of face detection, so, unless you want to implement your own method because the ready one fails, this should be a non-issue. See:
Show[img,
Graphics[{EdgeForm[{Yellow, Thick}], Opacity[0],
Rectangle @@@
FindFaces[img = Import["http://i.stack.imgur.com/cSwzj.jpg"]]}]]
Supposing the face is detected correctly, you can expand/retract its bounding box to match the size you are after.