It used to be in the "good old days" (iMovie 9 and prior) you could take a screen shot using the clip menu 'Add Freeze Frame' then reveal that frame in the Finder. Well... for some unknown reason Apple has removed this functionality. You can still make a freeze frame but this action no longer creates a still JPEG file.
Your choices now are:
- Use the Mac's built-in frame grabber (Command-Shift-4) or similar program like Snapz to capture the image
- Use MPEG Streamclip, find the frame you want and Export that Frame
If you think this is stupid and this option should be added back to iMovie please contact Apple here.