One way to teach them, is to rationally explain that you are not going to insult your parents. If they keep insisting, stand up and walk away. When faced with obstinant rationality, people just don't know what to do.
I'm assuming they're taken you to a 'children's' photographer. You might want to ask your parents to take you to one who doesn't specialize in children - especially if it is they who are insisting you be photographed in the first place.