You tube generates a thumbnail of the video uploaded and is shown as default before it starts playing. In case you want to grab that thumbnail and want to use it somewhere here is small trick.
Get the YouTube video id
You tube video id can be found either in the direct url they give e.g 5qt-7zEJwKM ( Tha string after “v=” ) or in the embed code where it says <embed src=”http://www.youtube.com/v/5qt-7zEJwKM .
Get the thumbnail image of the YouTube video
You tube sores all these images under their sub domain IMG. Copy this url where Video ID is the one you just grabbed using above step. paste it in your browser and hit enter.
e.g You can see this link http://img.youtube.com/vi/5qt-7zEJwKM/default.jpg directly.
This trick was explained by Scott Klarr. He also explains how to get the Google Video thumbnail which is little more complex. I would suggest you to read his post on How to grab thumbnails for YouTube and Google video