just as a reference:
YouTube supplies thumbnails for a given video.ID in this way:
http://img.youtube.com/vi/MKczB_9LChY/0.jpg
http://img.youtube.com/vi/MKczB_9LChY/1.jpg
eg:
img.youtube.com/vi/VIDEO-ID/0.jpg is a 320x240 image
img.youtube.com/vi/VIDEO-ID/1.jpg is a 130x92 image
using a custom field in WordPress (video-ID) with value 'videoID' you can create a function to display the thumbnail of a YouTube clip. This will create dynamic YouTube image thumbnails on the fly!
For VIMEO clips there is help supplied here: http://www.vimeo.com/forums/topic:5913#comment_1224297
Interested? Ask for!!