VLC will allow you to do it. You can usually use the Command + Shift + 3 on the keyboard and it'll take a picture of your video. Or use Command + Shift + 4 and hit the spacebar, this will allow you to take a screenshot of the selected video. To take a snapshot of the dvdplayer, you'll need an 3rd party app or plug in.
Since apple will not allow the grab utility to be used whilst playing any dvd.
You can try DVDsnap
http://www.tool-forcesw.com/shareware_dvdsnap.html
or you can use SnapNdrag,
http://www.yellowmug.com/snapndrag/ it's free and it does a good job.
If you want to do it in the dvd application itself, it's really easy. You'll need to create an new apple script in ~/Library/Application Support/DVDplayer/Scripts. You'll want to use this "code" in the script editor: do shell script "screencapture -iW ~/Desktop/dvd-window.png" (with out the quotations).
Click on file then save the script in ~/Library/Application Support/DVDplayer/Scripts You can name the file what you would like, I name mine DVD Screenshot. Then all you have to do is open the dvd player application, and play a dvd, if you find a frame you would like to take a screenshot of, click the "Scripts" Icon (it's the icon that's by "Help") And go down to DVD Screenshot (or whatever you named your script) Click it, and then click on the dvd player screen and it'll take a snapshot to your desktop.