LDR_YouTubeThumbnails.user.js

// ==UserScript==
// @name    LDR_YouTubeThumbnails
// @include http://reader.livedoor.com/reader/*
// @version 0.1
// ==/UserScript==

(function(){
function source() {
    window.register_hook('before_printfeed', function(feed) {
        if(feed.channel.link.indexOf('http://www.youtube.com/') === 0) {
            feed.items.forEach(function(item){
                var id = item.link.match('v=([a-zA-Z0-9\_\-]+)&?')[1];
                item.body += '<img src="http://i.ytimg.com/vi/' + id + '/mqdefault.jpg" />';
            });
        }
    });
}
var script = document.createElement('script');
script.type = 'text/javascript';
script.innerHTML = '(' + source.toString() + ')();';
document.body.appendChild(script);
})();