We had Belle recently. They let you take your own picture if you want, then the pro steps in and takes the one they will use to make prints for you. They give you the prints before you leave the restaraunt. Then you have to carry them around in Epcot... Maybe they can be sent back to your resort or to guest services.
The professional prints are nice, but I like the pictures I took at the table a lot better. And mine are digital. Although I could scan theirs if I wanted to once I got home.
Here's some tips:
If your table is against a wall (ours was), put the person taking the pictures in a chair with back to wall. Put children in the party across the table, so that the Princesses can interact with them easily.
If your camera has a video function (mine is not a full fledged video camera, but it's a digital camera that is able to take videos) turn it on when the Princess leaves the neighboring table and is about to approach your table. Try to get the kids in the image. The reactions on their faces, especially if it is their first interaction with characters, is priceless.
Get an autograph book ahead of time.