Dear Leslie,
I think that every one of us has at least occasionally, had the feeling that "Difficult client" is just redundant in the Department of the Redundancy Department
You might look at three-part pricing structure - several hundred down for the photo sitting, 40% or so in order to start the painting, (both non-refundable) and balance on delivery. When I handled all my own local business, I used to break up the payment method this way, and even though I told the client the % down was non-refundable, I would refund the $ anyway if I couldn't make him or her happy.
You're right, there is nothing worse than a client who has a portrait on the wall, and who is ready to tell any and all, "I paid this artist, and got this horrible painting". The second worse thing is to have your client say," I paid this artist and hated the painting and she kept my money and the painting."
I realize that my approach is very un business-like, but at least with the local people, I was out a few hours of time, and had some $ to compensate for the photo shoot. Now that I work mostly out of state with brokers, they handle all the contract and $ issues. Even so, I recently had a situation where I had already conducted the non-refundable sitting, and realized that there wasn't any way I could please the client (having just painted the older child), and I sent the whole deposit back, no questions asked. Writing that check took a huge weight off my shoulders.