From what I have read, the issue was with a service called SnapSaved. They reverse engineered Snapchat's API, found some vulnerabilities, and it allowed them to archive what were supposed to be ephemeral pix and vids. So there's two problems here: Snapchat wrote a buggy API (for which they have been criticized) and some users wanted permanence, thus creating a demand for SnapSaved and negating the purpose of Snapchat. I get the impression that were one party to send an image to another and the second party is using SnapSaved, the first party might never know.

I came across this today which talks about some of what's going on:
http://www.wired.com/2014/10/the-snappening-is-not-your-fault/