There are professional photographers at each of the 4 parks.
They will take your picture on Main St. at MK, on Hollywood Blvd at MGM Studios, in front of the Tree of Life at Animal Kingdom, and in front of (and behind) Spaceship Earth at Epcot.
The photos can be viewed and purchased at least 2 hours later at the "camera center/photo shop" at each park (MK: Expo Hall, DAK: Garden Gate, Epcot: Camera Center, MGM: Cover Story) The photo shop is located at the park entrance to the left as you are exiting (almost exactly the same location at all 4 parks).
It costs nothing to get the photo taken. The photographer will give you a card with a number on it and you can decide later if you want to buy the photo.
The photos cost between 12 and 20 dollars... I haven't purchased one in so long that I don't remember exactly (anyone know off hand?)
There are also photographers at some of the character greeting areas that send their photos to the same location.
