Web App similar to the iPhone App Snapchat (secretify.herokuapp.com)

I've hacked together an app that does something similar to the popular iphone app Snapchat. It shows an image temporarily, after that, it is gone forever. I'm using RoR, Javascript, Ajax, HTML, CSS.

I'm still stuck on how to block screenshots. I think using key events I can block it on a mac but not sure how to block it on a windows computer.

Any feedback would be great :)