As a mobile dev, how would you stop users from taking a screenshot? My girlfriend uses snapchat with her friends a lot and if she is quick enough she can grab a screenshot of the snap, I assume other people are doing this often.
In this case, making sure the file isn't on the device longer than it should be.
Keep in mind, when it comes to extracting images, you can't stop a sufficiently informed or sufficiently determined end user, because all of your code runs on a device that's wholly in control by the end user. All you can do is basic due diligence.
There is the side problem where the attacker isn't the end user (e.g., malware attempting to extract private images from a phone). At the end of the day though, there is squat all you can do against code that has root.
Contrary to some other platforms (to my knowledge, most notably Android), iOS apps are very sandboxed.
Edit: I have personally confirmed this on iOS. According to a commenter below, this is also true on Android.