Um. He doesn't have to quit nor can they legally fire him since the photography is extra. He isn't refusing to do is actual job.
I'm not saying that he HAS to quit, or he runs the risk of being fired. What I am saying is employment is a two way street. One challenge an employer faces is finding talented employees that can make them money. The more money you can make for an employer the more valuable you are to them. If an employer knew they were at risk of losing a proven money maker, they would/should be concerned. Now, because they hired a photographer, the OP MAY no longer be as valuable to them as he was before.
Think of it this way, let's say I am lucky enough to find a very talented retoucher who makes me a lot of money, then find out they are also very good at accounting, so I ask them to handle a lot of my accounting duties rather than having to hire someone else to do it. I give them no additional compensation and no recognition. If at some point my talented retoucher, explaining the additional value they are adding to my bottom line, asks for additional compensation, or at least some kind of recognition and I refuse, I should know that I risk, at some point, of losing my talented retoucher. It's a business tightrope that both sides walk. Life is full of decisions.