I imagine it would be the same as any screen capture. Just go to the picture that you want and hit the capture button. I had a problem capturing earlier, but that is because I had the OEM version of Cyberlink PowerDVD, rather than the retail "XP" version, which can capture. Rambling aside, just treat it as you would any other screen shot. You could even try to pause the DVD on that specific photo, and it should work. Good luck.
EDIT: If you are having trouble, please list your computer DVD player on this site, and help should be easy to find.