It's not exactly a new technique but it's effective for most super targeted attacks, honestly it seems if you were this inclined to be able to get a specific app on the users phone, you might as well just work off the Android app you've already gotten delivered to the users phone. Like Facebook.

Throw a privacy notice to the users "This app will take periodic screenshots of your phone" You'd be amazed how many people will accept it.

> Did you release the source code of Pixnapping? We will release the source code at this link once patches become available: https://github.com/TAC-UCB/pixnapping

It's not exactly impossible to reverse what's happening here. You could have waited until it was patched but sounds like you wanted to get your own attention as soon as possible.