You don't know where it could end up.
Thousands, even millions, post so-called pictures of themselves on Facebook, for instance. Even on JUB people admit to using pictures of others on Facebook, let alone here. If it were impossible for a pic posted in a safe environment like JUB to never leave it, fine.
But that's impossible. For all you know, your mug could end up on a pedo or neo-Nazi site or be used on a terrorist site or in a death-threat post or email by someone who knows never to post his own pic — or just through plain old malice. Someone, somewhere, might recognize you as being on a site you never visit, whether or not it is repulsive to you. All that's needed is to attach your name, then everything you are is blasted all over the media.
Maybe you'd never know it's there, but would you want your pic on such a site even if you never discovered it? Perhaps, though, you suddenly do discover it because of the grave results.
It's not as though it's never happened.