Wordpress XML-RPC and featured images

I'm currently developing a plugin for a client that takes an xml feed hourly and posts it into wordpress and I'm having trouble sending the featured image to the post.

I can post to wordpress fine but all my attempts to post the featured image have failed.

<?php
class XMLRPClientWordPress
{
var $XMLRPCURL = "";
var $UserName  = "";
var $PassWord = "";
// Constructor
public function __construct($xmlrpcurl, $username, $password)
{
$this->XMLRPCURL = $xmlrpcurl;
$this->UserName  = $username;
$this->PassWord = $password;
}
function send_request($requestname, $params)
{
$request = xmlrpc_encode_request($requestname, $params);
$ch = curl_init();
curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
curl_setopt($ch, CURLOPT_URL, $this->XMLRPCURL);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 1);
$results = curl_exec($ch);
curl_close($ch);
return $results;
}
public function create_post( $title, $body )
{
$title = htmlentities( $title, ENT_NOQUOTES, 'UTF-8' );
$content = array(
    'post_category' => array( 18 ), // my category id
    'post_type' => 'post',
    'post_title' => $title,
    'post_content' => $body,
    'featured_image_url' => 'http://www.geekologie.com/2009/02/18/scary%20clown.jpg',
);

$params = array( 0, $this->UserName, $this->PassWord, $content );
return $this->send_request( 'wp.newPost', $params );
}
}
$objXMLRPClientWordPress = new XMLRPClientWordPress("xxxx/xmlrpc.php" , "xxxxx" , "xxxx");
$objXMLRPClientWordPress->create_post('Hey Chloe','Hope you like the clown'); 
?>

Is what i currently have, i've been reading the wordpress patch tickets on this issue but can't seem to figure out how to actually use the new features

Thanks

It should be something like this