When you come into Magic Kingdom and start walking down Main Street toward Cindy's Castle, there are photographers everywhere. They'll stop and ask you if they can take a picture of you and your family. Also at Animal Kingdom when you go across the bridge and the Tree of Life is directly in front of you, there are photgraphers everywhere. I can't remember where they were at Epcot and MGM but I've seen the pictures from other families and they always try to get the main focal point of each Park behind you. Example---MK, Cindy's Castle, AK, The Tree of Life, etc....I would think at Epcot they'd want the World Ball and at MGM, Mickey's hat.
After they take your picture they'll give you a piece of paper w/a number on it. I think they're ready w/in 30 mins and can be picked up anytime. The really cool thing is if you're staying onsite, you can ask them to forward to your resort. On our first day at MK, we didn't know our room number yet and they said they'd have it sent to the gift shop. Picked it up that night and didn't have to worry about packing the pix around everywhere, it getting wet, wrinkled or being lost. Awesome service! Don't do like we did tho and be so tired when you leave to forget to stop and look at your shot. My understanding is they'll keep the pic on file for 24 hrs but after that they're gone. I might be wrong on the time but I think that's correct.