Part of the head is already textured. Go again to "Master Projection" to correct the image. Select "Simple Brush" tool, choose alphamap named "Brush 01", select "DragRect" stroke and "Import" some skin photos. Set "RGB" to [100], turn off "ZAdd", put textures on model and repeat the process, unitl you have the whole head painted.

Now ZBrush will generate the details. Go to "Tools -> Masking", press Int, turn off texture displaying for a while. Set "Deformation -> Inflat" to about [30]-[40], "... -> Smooth" to [10] and uncheck "Masking -> View Mask"