Look4Faces is a desktop application that lets you scan a QuickTime-readable video clip (M4V, MOV, MP4) to find frames containing faces. As a result, Look4Faces will list all frames with one or more faces. You may then select one under the Pictures window and save it as an image file, or select several frame shots with faces and let the application export them to a folder you specify.
Look4Faces comes with a built-in video player. You can click on a particular screenshot with faces to locate the exact frame over the video player. Furthermore, you can take a screenshot at any play head position just by clicking on a button.