One thing I noticed was that the characters will take as much room for their signature as you give them. We forgot our autograph book one day and they signed just fine on a small (maybe 3" x 4" paper), but when we had our 5x7 autograph book they used the whole page to sign.
If your planning on adding pictures I suggest you somehow block out the area you intend the picture to go or they may sign right over the top of it.