Video upload

I just uploaded my first video. I tried two different video formats, Apple's .mov and MPEG4. The MPEG4 file was larger (12 vs 3 MB), but I could not see any difference in quality and how they played. While the video upload system was processing the MOV format file initially showed at YouTube as having a zero duration, but it eventually came through with the correct time. I'm left wondering how to control which frame of a video is used as the static image shown for the video. According to this there is no way to select the thumbnail image besides playing with the content/length of the video. The trick is to have a "cool" image as the middle frame of the video.

Video rendered from the MPEG4 file