by the look of it large pictures are being moved up and down forth and back , etc ; what you see is under the area of a mask which covers up the large pictures in movement ;
try to do the same with a picture on a layer below a mask ;
I think the rest is tuning up the movements to look like grandamarathon ;
don´t know if it means something to you ?
the site has html part and a flash movie as well ;
right click over that marathon movie . you will see that is a flash movie on which a mask was used to hide the movements (tweenings ) of large pictures ;
OK…I think I get it. I am actually moving the photograph but the movie itself is smaller than the photograph, so you don’t actually see the edges of the photograph moving, just the middle of the photo!!