I think applying the principles shown here should solve your problem:
var xml,
$xml,
data,
$data,
imgSrc;
// Parse the XML using jQuery
xml = $.parseXML('<description><![CDATA[<p class="image"><img width="250" height="83" src="http://www.mydomain.com/picture/pic01.png" class="post-image" alt="Post 01 Image" title="The Demo Post" /></p>My Content]]></description>');
// Create a jQuery object from the parsed XML
$xml = $(xml);
// Select the data
data = $xml.find('description').text();
// Create a jQuery object from the data
$data = $(data);
// Select the img src attribute
imgSrc = $data.find('img').attr('src');
And here's a fiddle to show it working:
http://jsfiddle.net/xonev/Vj4W9/.
