As was mentioned in the other thread. Make sure no money changes hands. (or anything of value).
The FAA is quite strict on "illegal charters", "for hire flights" and other situations where money is involved.
However, it's absolutely no problem to take aerial photo's (within reason of course). Hand the pilot your camera. Enjoy the results.
You can't pay your friend for a flight. You could split the costs. But, there is a new ruling on that one. You both must have a similar interest in the flight. For example, Bob and I want to fly to Oshkosh. We can split the costs in my airplane. Example 2: In your case, you want pics. Your pilot does not. If you split costs, your pilot would be, under current interpretation, violating the law. It used to be less strict.
Who cares? You could do this for years and never get caught. However, the FAA will revoke his ticket without delay or recourse, on the spot. In fact, the way these guys act now, I'd be surprised if a pilot caught doing this ever fly's again.