Capture still image from DVD

I would like to create snapshots from a DVD and save to JPG or other format file. I tried Command+Shift+3 and a utility called SnapNDrag. I get a grey screen, whether the DVD is paused or playing. I'm guessing this is some kind of copyright protection. I'm not ripping off someone's work, this is a DVD made by a friend of home movies.

If I can't do this with the DVD player, can I somehow extract the content as a movie file? I think I could then use iMovie to grab stills, no?