I recently began spinning dials to recreate actual facial photo images.  My work flow is to load the portrait onto a flat square, use the scale x & y to size the image to the same resolution dimensions, then resize the square overall to match the model.  In this way I can load different angle images to the same square and move it around the model.  The attached image is a WIP of Christian Bale modeled to M4 using morph dials.  As modus0 mentions, final refinement can be done with the morph tool.