Make a movie clip. On the timeline, place the squares, which can be another movie clip with just a white fill in it, over the area of the picture. Place them all in frame 1 and remove them along the timeline of the movie clip so as it plays, the squares go away and disappear. Now put the squares movie clip inside another movie clip that has your picture on layer 1. Add the squares mc on layer 2 and turn it into a mask layer.