To do this, create a Thumbnail image on the web and upload it, then put inside a Tag Tag Head.
Sample
<html>
<head>
<link rel="image_src" href="http://www.yourdomainname.com/image.jpg" />
</head>
<body> . . . </body>
</html>
Reference : http://www.memo8.com/toolkits/archives/623
