See http://blog.prashanthellina.com/2008/03/29/creating-video-thumbnails-using-ffmpeg/
ffmpeg -ss -4 -i test.avi -vcodec mjpeg -vframes 1 -an -f rawvideo -s 320x240 test.jpg ffmpeg -itsoffset -4 -i test.avi -vcodec mjpeg -vframes 1 -an -f rawvideo -s 320x240 test.jpg