If they ask for it, give it, even if they call it optional. It looks worse to some not provide one than to show yourself to them. It's not like you can get into their school without them ever seeing you, so you might as well pull that trigger. Not happily providing a picture conjures images of something kept in a bell tower or locked in the basement.
Look at the admissions process a lot like dating. You aren't going to get a first date without a pic.
It is to make sure to screen out potentially ugly doctors...
But in all fairness, it is usually for post interview so that they can put a face on the application, so you are not just a name and number but a person...I think..