Bryan,
If you're painting portraits specifically as portfolio pieces, you have the opportunity to use only the best photos. There's no client urging you to use their old snapshot. Make the most of this opportunity to do your best work.
Do a search for posts on "value massing" to get a better idea of what Karin's talking about. It's a new concept to me. I learned about it here.
My only comment on the painting is that the hairline is too sharp. I haven't seen the photo, but I'd imagine there's a subtler transition between forehead and white hair.