Glenn,Copyright belongs to the person that does the work. If you are using a reference photo supplied by the client, you should get permission to use the photo for your work - which would be a considered a "dirivative" of the original photo.
It would also be a good idea to get a model release from the subject which states that you have permission to display the portrait in progress and finished.
Copyright laws are very missunderstood by many in the art world. I suggest everyone research the subject before they begin selling or displaying their artwork. A good place to start is a book entitled " The Artist's Friendly Legal Guide" Published by North Light Books.
J. D.
J. D. Hillberry
http://www.jdhillberry.com