Move your red start marker to the position in your movie that you want to make into a picture, and then go to your menu bar, click "File", then "Export Movie", then "Single Frame as BMP" or "Single Frame as JPG". BMP is better if you plan to import it back into the video, because it's not a "lossy" format, it will still match the appearance of your video, while JPG probably won't look right mixed in with the rest of the frames.