Just counted for you....50 pages (100 sides). We've never filled an entire book -- and we try! Last trip, my DD got 46. My older DD may have gotten a few more. You could have a character sign one page (right hand side) and put a photo next to it, on the left hand side of the book. It would be on the back of another character's signature, but I think that would be ok.
Like this:
photo / signature
Hope that helps.