I take my own pictures of a client and will not paint a portrait from one that someone else has taken. These are my personal rules and I have never had a client question them.
But there is an exception...that of a posthumous portrait and an existing photo of that person is all that there is. Needless to say, these kinds of portraits can be very difficult as the lighting is seldom good enough to paint from.
In order to produce a posthumous portrait, I take the photograph I have been given and find a model of a similar body type and age. I then photograph the model in similar surrounds (clothing, props) in my ideal lighting. See
http://kcwells.com/technical2.htm
I then use my photograph as a reference to paint all but the features from...and so far, this has worked very well for me.