Applications like this is why SnapChat is not as save as people think it is. While the service does do a decent job for displaying a photo for the limited time it is supposed to be available, it is not impossible to capture the photo. Apps like this make it possible to capture the photo and hold on to it forever. There are plenty of other ways this can be done without the aid of these apps. Things such as Screen Captures on your phone. These are relatively fast and very easy to do, making it possible to capture an image even without the help or 3rd part apps. As I have said in the past, don't send it unless you want the entire world to see. Once something goes out on the internet, it's public and anyone can get it.
If you think your safe, because little Johnny would never capture an image from SnapChat on you, well maybe your right little Johnny may not. But what about the guy who has hacked the wireless network your on? or little johnny? or some piece of the network between you and little Johnny. All it takes is a packet sniffer to capture the information on the wire, and it can be re-assembled and made viewable.
Once it leaves your control you no longer have a guarantee of it being safe or private. In fact in some cases even while you think you're in control, your really not. A small simple virus that uploads your photos to some server for someone to get. Bottom line don't every think anything you send out is private.