The only way I know of capturing a screen image is to press prt scrn once. From there, exit (or minimise) what is on there, and open Paint in windows accessories. Once there, select paste, and this will insert what you have captured. Save it, and then carry on with whatever.
Sometimes pressing "Print Screen" doesn't work and when you paste the contents of the clipboard into an image editor you see the movie still in action instead of one frame of it.
If that happens then use a media player that has a screen capture function built in : the only one I've used is Power DVD.
Failing that, open the movie file with TMPGEnc, click File > Preview and then take a screen grab of the preview screen.