If you are playing it Quicktime, all you have to do is hit the 'Print Screen' button on the keyboard, and then paste it into a photo editing program (Windows Paint if you have nothing else).
Alt + PrintScreen will just screen cap the active program, not the entire desktop, so that can make the files smaller.
Doesn't usually in players like PowerDVD and such, but it works fine for Quicktime trailers. (Assuming a PC of course, not sure what to do on a Mac)