I've discovered a great tool for adding missing thumbnails to videos.

Puush is a free screen-sharing program with online hosting. When you have it installed, you can simply Ctrl+Shift+4 to capture any rectangular area on your screen (for instance a video frame), which is then instantly uploaded as a PNG image file to your online account. At the same time, it copies a short URL to your clipboard, which you can paste into the image URL field of the thumbnail adding page. And thus, in two short steps, the offending video is fitted with a splashing new thumbnail.

Of course, it is also of great use if you do any kind of work online where it's necessary to instantly show others what is happening on your monitor. Other keyboard shortcuts are available for such things as sharing your entire screen or a certain open window. You can also upload any kind of file and get the same practical URL copied to your clipboard.

It is available for Windows and Mac at http://puush.me/

Give it a try!