Well, an easy way to do it would be to open the video, pause it on something you like, press printscreen, and paste it into an image editing program.

*edit*
Actually, the trick to that is, when you've paused it on the frame you want, minimize the player, restore it, and move it around. If the video inside the player is moving around right with it, it can be captured with printscreen. If it's sort of "lagging" behind the window when you move it around, minimize it and restore it again. It's weird, but it works.