In the page where you can selected from your archive of profile pics - you can selected which ones to be added to a random pool which will now be shown in your profile picture.

What does this mean? As an example, you can click "Add Into Profile Pic Pool" and it will now be in the queue to be randomly displayed along with others that you have selected every time a user views your profile.  If you want to remove a pic from the pool simply click Remove From Profile Pic Pool.  If you want to scrap the whole thing and go back to only displaying 1 static profile picture, click Use This Pic which will replace the whole pool with that one single pic.