There is a photographer on site that you can arrange to have pictures taken with. They are usually wandering the resort taking pictures as well, usually in the evening. We have several pictures that they have taken.
Just check in at the photo shop and they can set this up. We saw this happening almost every evening while we were at CTI. The photographer even took our photo while he waited for his scheduled couple to decide their pose. Sorry, we don't have it on-line to post.
You don't have to set it up before you go or anything (when you are at the resort is soon enough) but CN does have a photographer. I think you can set up a time for pictures (not sure) but he is around through out the day taking pictures between weddings of couples on the beach, at the pool, etc. & is always around at sunset & dinner time. We wanted pictures on our anniversary & just flagged him down on the beach at sunset & told him. He took probably a dozen or more pictures. There are 3 of them in my album - Couples Negril 2007 - if you want to look at a sample.