to take screen shots of a DVD using DVD Player you'll have to find a 3rd party app, there are some available on versiontracker.com but i haven't tried any out.
You could always rip the dvd with handbrake first then do screen shots from the Quicktime file.
Export as image sequence for animation stills for the gif.
SnapzProX that can do it if you want to pay for it (unless they have a trial for it). I have used and like the free CaptureMe app. I use this for pulling dvd shots for use in Flash or Photoshop for my art Give it a search on Version Tracker as that's the last place I found it.
Open the terminal (applications/utilities) and type:
screencapture -ic
After you hit enter press space and a camera icon will appear instead of the mouse pointer. Float it over the window you want to capture and click. Now open your fave image editing app and paste