Like Old said, set up a movieclip with a different picture on each frame of the movieclip, let's say ten frames for this example. Give the movieclip the instance name 'photos'.
When the site opens have this code on the first frame:
frame=Math.ceil(Math.random()*10);//creates a random frame to go to
photos.gotoAndStop(frame);//sends the photos clip to that frame
This might not always be exactly what you need as there's a chance that somebody would get the same photo more than once (since it's random).
You could develop it further adding in some extra code that 'remembers' what you saw last time (using a shared local object) but I think the above idea is a good start if you're not really used to the program yet.