And while I am definitely in the camp encouraging women to breast feed their infants, and while I would never try to establish a hard and fast rule by when the child should be weaned, I suspect that when he's old enough to walk over to you and undo your blouse, it's probably time! (Pun intended.)
In order to avoid massive amounts of therapy, I suggest that momma show the lad these pictures when he is a little older, before his classmates get a chance to see them. If you thought the "birds and bees" talk was hard, how do you begin the "Mommy's picture breast feeding you is all over the Internet" speech?