I know you can use an outside photographer, but I called (as a bride) when my fiance and I first started looking for a location and they hassled us about it. I don't think they have much legal ground to stand on with that since any photographer with a work permit can work in the Bahamas, but that doesn't mean they aren't going to try to persuade couples otherwise. They do consider outside photographers a wedding guest, and require that they stay at Atlantis. Another interesting thing they told us was that ALL of your wedding guests had to be staying at Atlantis, and that Comfort Suites guests aren't permitted. However, we saw plenty of weddings out there and I didn't see them check for the Atlantis bracelets... so either I was told an outright lie or they don't have a way to control it. I would guess though that they simply don't check for bracelets before the wedding. Here's a link to the FAQ section for Atlantis.
I wouldn't bother dealing with Atlantis though. It's too much of a hassle and there are too many other great places to get married. They lost our business already and we found a much cooler venue on the opposite side of Nassau!