YouTube now has a feature that allows you to choose which of the 3 thumbnails they generated to be used for your videos.
http://www.youtube.com/blog?entry=Jwou-luoqEw

You can't upload your own yet, but it's better than having a blank (black) thumbnail I suppose. :)

See it in action: http://www.youtube.com/my_videos Click on Edit Video Info

>You can't upload your own yet
of course, you will not see that soon. Imagine the spam abuses...

> Imagine the spam abuses...

"this video is brought to you by ..." :)

They've actually always had the three thumbnails...I had tweaked some Javascript I found to make it so you could see thumbnails for clips, and then on a mouseover the thumbnail would cycle through all 3 available thumbnails.

The code is in action here:
http://symbii.com/mockdrafts.php#YouTube

Here's the Javascript code:

function Animate(c) {
var b,a,d=document.getElementById(c);
d.src=(d.src.substr(0,b=d.src.lastIndexOf('/')+1)+
((a=d.src.substr(b++,1))==3?1:++a)+d.src.substring(b));
swapit=window.setTimeout("Animate('"+c+"')",600);
};

function DeAnimate() {
window.clearTimeout(swapit);
};

And then you call it like this (just call Animate with the img id):
<img onMouseover = "Animate('youtube1')" onMouseout="DeAnimate()" id="youtube1" src="http://img.youtube.com/vi/1dmVU08zVpA/2.jpg">

The last part got cut off, but after the id you'd have the source of the youtube thumbnail, i.e. http://img.you tube.com/vi/1dmVU08zVpA/2.jpg and that's all that's in the img tag.

Also, I think you can manipulate what's in the thumbnail...I thought I remembered someone saying the (main) thumbnail comes from the exact halfway point of the video, and some people would even put in split-second screens in order to manipulate the thumbnail. I guess that may have changed recently.