This is a nice blog Squidoo post about changing the “poster frame” of a YouTube video. I call them thumbnails, but poster frame works. The Cro used my video to show how you can see the thumbnails on a video, and generate a higher resolution one by changing some digits in the URL.
Recently YouTube made a change so that if it finds a still image in the frames surrounding the center frame it will choose an alternative one. This can be addressed by giving your center-frame image a “zoom effect.”
I actually work hard to ensure that my center frame is representative of the video since I know how important that thumbnail is to views. It’s a shame YouTube doesn’t allow creators to select their frame like Revver and Metacafe facilitate.