Hello. I made my reservation for the photo session (40 minute one) in January for our May trip. I called Aulani directly to make the reservation. You do not pay for the session in advance, the lady told me the instructions. I believe you pay for the session earlier in the day and then you show the photographer the receipt at the time of the session. You are entitled to a 10% discount if you are a DVC member. I asked about being able to go to different spots during the session and I was told that the photographers don't do that. The session is mainly set up on the beach, but they may be able to do some shots with some greenery in the background, but I definitely got the impression that you cannot walk around the resort and have pictures taken where ever.
She also said that if you purchase a photo session, that you can then purchase the photo pass length of stay photos for $50, which is half price from the normal $99 price tag and that you can wait until the end of your stay to decide if you want to purchase those pictures.
Lastly, I am under the impression that they are photopass photographers, maybe just a little more skilled (or seasoned) than the other photographers they have throughout the resort.