First, go to the scene on the DVD that you want to capture, and hit 'Pause' on the DVD player to ensure you are stopping exactly where you want to be. Then, in the DVD Player menu bar, go to Controls > Use Current Frame as Jacket Picture. This captures the frame in the movie you are on. Then click 'Stop' and your selected frame should come up - no matter where you stop the DVD.
Take a screenshot (Command-Tab-3) of the jacket picture and it will appear on your desktop like any other screenshot.