The pre-pay thing is a relatively recent phenomenon. I know when Hayden Christensen did Fan Expo Dallas a few years ago he was prepay only. There was no just getting in line. In my experience most guests still do it thr old fashioned way and I think some may do hybrid, where you can prepay or do the line. Some are switching to just prepay though.
Photo ops are a separate thing. They leave their table for a period of time to go to the photo op area with a photographer then you go and get a professional photo taken during the allotted time.