Copy/pasted from the FAQ.
Q: Does the Screenshot button work?
A: No, it doesn’t. The button is a feature that was in the original Facebook version of the game, but it has no use on Kongregate. If you would like to take a snapshot of something in the game without it pausing, there is a simple way to do it without having to download special software. Have whatever you want to take a picture of on your computer screen. Hold the Alt key and press the Print Screen button. Then, open up a new document in Microsoft Paint (the basic painting program on your PC), and go to Edit → Paste. Then edit and save the picture.