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.
Yup, your right, just reverse what I said then, add them to the timeline. On the other hand, you don't even need to use a mask, you can make one square movie clip first, with a fade out effect on it's timeline and a stop at the last frame. This would fade in the picture, in squares.
Here, get this fla and check it out. It uses some code and a movie clip as the mask. I didn't finish the squares, but you can see what to do to finish it.