Shutters lines up photos on racks on the walls and groups them by event - ie embarkation photos, character breakfast photos, photos with characters, formal night, Palo,
Castaway Cay, etc.
They have to rotate photos periodically to fit new ones up. By the end of our cruise embarkation photos were outside Shutters and there was a pile of other photos on a table that you had to look through.
When you buy a package, you are given a card with numbers on the bottom representing the number of photos you have bought/left. You can upgrade your package at any time and you can exchange up to three photos.
Stacking photos is a way to keep all of your familys' photos together so you can sort them at the end of your cruise. A few word of warning - there is no guarantee that the photos will still be where you put them when you come back, also Shutters is VERY busy toward the end of the cruise. You are better off buyinf them and taking them to your cabin as you find ones you like - of course that will probably result in you buying more pictures.
Another tip, bring a photo mailer to keep your photos flat on the way home.
We spent a lot of money on photos, even though I said I wasn't going to. They are worth the money as far as I am concerned.
Hope this helps.
Denae