Anyone who has ever been to one of my presentations is familiar with my stance on kids posting pictures on any online medium (Facebook, MySpace, etc.) or sending photos via their cell phones. One of the dangers I try to make parents aware of is how "leaky" the digital world is. Once a picture is posted it can spread to thousands of different sites through innocent exchanges by friends and peers. The concept of the "toothpaste being out of the tube" is the analogy I often make. When a child sends or posts a picture online, it becomes the property of the world and you can never take it back. And what's worse, since many pictures come from social media sites, they have names associated with them - names that can be indexed by search engines and names that can be searched by anyone.
CNN ran a great article about this and the author does a better job articulating it than I can.
Here is the link: http://www.cnn.com/2011/TECH/social.media/04/08/photo.privacy/index.html?hpt=Sbin