Each time you divide the mesh, add bolder and precise details. Select "Pinch" tool and make wrinkles on the forehead, at the lips and near the chin. With "Inflat" fill spaces below them. You can "Smooth" the sharper places. Your 3D View should look like this:
Save the head for safety. ("Tool -> Export", DON'T save it as a Document!) Let's paint the texture. Turn off "Edit", press "Undo" (Ctrl Z) to erase the model, load the reference image, then press "Crop and Fill" and again load the head model. Press "Edit". Now use the "Move" and "Scale" to match it to the reference. Create new texture in high resolution, load "Image Plane" script (you can download it and get help from zbrushcentral.com). Go to "Master Projection", keep the "Color" option turned on. Move the texture till you have the proper effect, then press again "Master Projection" with "Color" marked.