I suggest you pick them up as you go, as they change them and move them so often you may never be able to find them again if you wait. In terms of exchanges, if you choose one early in the cruise and they mark it off your card of 10. 15 or 20, you may see a better picture later in the week. You have the option to then return that first picture and substitute the new one for it. You can do this up to 3 times. We made a stop at Shutters everynight to look and pick up the best ones of the day.
There were no specials the last night of our 11/15 Magic, only long, long lines and crowds so thick you could barely see the pictures. That is why I suggest picking up as you go.
Good Luck!