The app Snapchat, on the App Store, is an app that lets you share pictures with a self-destruct on them. You can only view the pics for X seconds. If you attempt to take a screenshot while the picture is showing using the home-power key combo, it will tell the sender you tried to take a screenshot.
What part of the SDK lets you detect that the user is taking a screenshot? I did not know this was possible.
I found the answer!! Taking a screenshot interrupts any touches that are on the screen. This is why snapchat requires holding to see the picture. Reference: http://tumblr.jeremyjohnstone.com/post/38503925370/how-to-detect-screenshots-on-ios-like-snapchat