Your working with less than 2 hours. It can take time to get the number of people the Freedom can carry off of a ship. If the ship docks on time and you are there at 11AM, assume at least 45 mins to get off the ship puts you at 11:45. You could get lucky and get off much quicker than that, but you never know. You also need to be at your excursion meeting point (normally at the ship) at least 15 mins before the scheduled time. That gives you roughly an hour to get your photos and get back to the ship. Its doable if you watch your time and hurry. Your other option would be to take the pictures AFTER your excursion. But you should be on the ship no later than 6:30PM, so that gives you even less time. If you are lucky, the ship will dock a little earlier and give you extra time.
As far as the ship giving priority to ship excursions, I wouldn't worry about it too much. St Thomas is a docking port. They are going to clear the ship and open the gangways and people will all start getting off the ship. I would just be ready to go and head to the gang way when they call that the ship has cleared customs and you can disembark. If this were a tendering port, then I would say you have an issue as 3500 people take a LONG time to tender and they WOULD make sure their tours get the first tenders. Yes, this is wrong but it gives them an excuse to charge so much for their tours. (The other benefit to ship booked tours are having things arranged for you and that if something happens while on a ship's tour that delays the return to the ship, the ship is supposed to "wait" for you. If you are on your own, they won't wait because they have no idea on where you are.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013