monline

Unfortunately Apple made sure that you cannot easily save frames from a movie using DVD Player. There's no such built-in functionality and the global shortcuts to save screenshots do not work if DVD Player is running. You have a couple of options though.

Most blog posts suggest you use an alternative player like VLC, which allows saving of frames. Some blogs tell you to use an alternative screen capture utility to save the contents of the window of DVD Player. But there's a third way: using the built-in screencapture command line utility. Just start it interactively (eg. screenshot -i dvd_frame.png) and point to the window of DVD Player. Of course you'll have to crop the image so it contains only the video area.