Image capture from itunes movie/ tv show purchases and rentals ?

I'd like to get screen grabs (preferably high quality images) from itunes movies for personal use. Is there any way to do this with Lion ? I have tried Snapz Pro already and just get the chequered grey and white image.

My macbook pro does not have a disk drive so this is purely for stuff from itunes which I believe cannot be played through VLC or quicktime etc. Any help would be much appreciated !