As long as you receive no more than a proportional contribution for the flight, then you are not breaking any rules. If it's just you & him, then he can pay no more than 50% of costs.
What he does with the photographs afterwards bears no relevance.
However, I don't know about his rights. What happens if he is not happy with the conduct of the flight, and as a result doesn't get the pictures he desired? Does he have any grounds for complaint considering he paid for half of the flight?
Thinking bout it, I suppose that he is paying for the flight, and not for the service of the flight.