Hat dir diese Antwort geholfen?
http://www.google.com/support/youtube/bin/answer.py?answer=58103&ctx=siblingSteht doch alles schön beschrieben auf den Hilfeseiten:
How do I change the default thumbnail for my video?
While we may add this feature in the future, there is currently no way to change the
screen shot that we capture from your videos for thumbnails.
What you could do in the meantime is edit the video so it is just a little
bit shorter and then
upload this new edited video to the site. YouTube will grab a different
screen shot of the new video because the length of the video is then different.
