Many DVD players on your computer have a capture function. Pause the movie at the particular point you want the frame captured and hit the capture key or menu item that allows you to do that.
The picture is placed into memory and you can then paste as a new file into your preferred image editing application. Keep in mind that DVD frames are only 768x480i so the quality isn't the greatest typically. Also be careful in how you use it so you don't have the MPAA chasing you down.
I did something similar when I took a scene from Animal House and stuck a bunch of classmates heads on it for a going away gift. It was pretty funny.