code:

// ==UserScript==
// @name        SA YT Links Thumbed
// @namespace   http://*.somethingawful.com
// @description Uses YT links to show thumbnails
// @include     http://forums.somethingawful.com/*
// @version     0.1
// @grant       none
// ==/UserScript==

var vidList = document.getElementsByClassName('bbtag_video');

for (var i = 0; i < vidList.length; i++) {
    var p = /^.*youtube\.com\/watch\?v=([a-zA-Z0-9_-]{11}).*$/;
    var vidId = (vidList[i].innerHTML.match(p));
    if (vidId[1] == false) continue;
    var image = document.createElement('img');
    image.setAttribute('src', 'http://img.youtube.com/vi/' + vidId[1] + '/hqdefault.jpg');
    vidList[i].innerHTML = '';
    vidList[i].appendChild(image);
}