Is there a way to get a screenshot / snapshot of a movie purchased on iTunes?
You can't take a screenshot of a protected video with the stock Mac tools.  Apple has built-in protection to prevent this from happening (if the video is not protected, then it's ok to take a screenshot).

The only way you'll be able to do this is to use a third-party video player, like VLC, or a third-party image capture, like SnapNDrag.