Hi,
I have been pulling my hair to accomplish the same thing. It is such an obvious feature (since there are so many people running video autoblogs with premium themes) yet it is either non-existent or very difficult to find.
The closest I’ve come is using:
1) the Custom Field Manager plugin that lets you to enter custom fields en masse in conjunction with,
2) the Youtube Thumbnailer plugin that then populates the created custom field with the thumbnail image URL from Youtube.
However, I am using the eVid Theme (by Elegant Themes) that will not show a thumbnail from an http URL (for reasons of bandwidth and slow page load times) and only accepts file paths.
So my question is, can someone who knows PHP tell me how to hard code Youtube Thumbnailer’s files so that it links to the upload media directory???