I put this code in a comment but wanted to share a better version here.
This is a code which needs to be modified and put within your code requirements. I created this code because the site was already using embedded youtube files but needed to show thumbnails as a teaser, efficiently.
First you need to install imagecache module to be able to resize the image realtime.
You can put this code in a theme function where you like to show the thumbnails and call this function within your .php.tpl file, if not already done.
$node variable needs to be as a reference to the function.
Considering the first array for the CCK content field name is field_embedded_link, you can change this by checking your field name. You can also uncomment print_r($field) and see the array.
<?php
$title_size = 45;
$field = get_object_vars($node);
$matches = array();
$match = '';
//print_r($field); // you can uncomment this part to show the fields
if (preg_match("/youtube/i", $field['field_embedded_link'][0]['value'])) { //
preg_match('/(http:\/\/www.youtube.com\/v\/)(.+)(\"><\/param>
$match = trim($matches['2']);
$fieldthumb = 'files/youtube/t' . strtolower($match) . '.jpg';