I wish you'd tell us how! I'd like to know...
There are several ways to do this. You could play the movie in the QT or other media player and stop the movie on the frame you wish to extract/capture. At this point you could screen capture the player window or a fixed area of the screen as a PNG file using the built-in "Command-Shift-4" action, set in/out points/trim the clip to include the single desired frame and save it as a single frame movie, use the QT 7 Pro "Movie to Picture" option to save the frame as a PCT file (with or without QT compression), or using a player like MPEG Streamclip (free) "Export Frame" option to save the frame as a JPEG, TIFF, or PNG file.
