[highphive]

The short answer is there really isn't a way to prevent screenshots. Snapchat has attempted to do so, but without much success. On iOS, it detects if either of the two buttons required to take a screenshot are pressed, and hides the image right away - preventing a screenshot unless the two are pressed truly simultaneously. In my experience, this works much better than on Android, where phones have a variety of ways to screenshot, and no great method exists to stop people from taking one.