Snapchat is supposed to not keep pictures and basically delete ("self-destruct") any pictures automatically. The ghostly cloud service itself was not hacked, but many megabytes of saved images have leaked... how?
Snapchat itself denies being hacked and blames an unauthorized third party which would have downloaded the "private" selfies to a website called Snapsaved which is accessible to the Internet, and was in turn hacked itself.
Snapsaved was being smart and reverse-engineered the API that Snapchat provides. This Application Programming Interface allowed Snapsaved to circumvent the “instant deletion” feature of Snapchat’s own mobile app, and save zillions of pictures to disk.
So, if you, friends, or underage family members have ever exposed any private parts to someone on Snapchat – there is a chance that this picture is now in the hands of hackers, and intimate photographs...could technically qualify as child pornography. Ouch, that gets you right in Federal pen.
Internet users, and especially teenagers should understand that anything you put on the Internet is there forever, and that is why you don't push your cell phone in your pants because it will bite you in the butt sometime later.