Trying to do a screen cap on quicktime m4v?

I have a feeling it's going to be something like copyrighted material but I'm trying to do a couple of quick screencaptures from my iTunes digital copy of a movie so I can have some costume references. All I get is a white screen capture. I used to use Snapz Pro X for DVD's but dont think it'll work on just a regular quicktime m4v?

thoughts or help is appreciated.

thanks