Apple doesn't want you post screenshots of the video that
you bought with
your own money. Their damned DRM prevents it.
I think VLC will be able to play the files, though, and you can take a screenshot from their... give it a try if you're up to it.
Alternatively you can get a program that strips the film of its DRM... you'd probably have to pay for one of those, though, and it may not be worth the trouble at all.