I wrote a php-redirect script, that sends the user to different pages, depending on which device they used to access the page.
Now if I post a link to the page (www.ibeat.us/i.php) on facebook, facebook doesn't show any of my images I have on this page. If I just add some "echo ..." lines, given the accessing machine is not running iOS, facebook writes those lines next to my link.
You can use the old image_src
<link rel="image_src" href="http://sample.com/my.jpg" />
Or the new Graph API og:image property
<meta property="og:image" content="http://sample.com/my.jpg" />
If you go by new one, remember to use the Facebook and Open Graph namespaces
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:og="http://opengraphprotocol.org/schema/"
xmlns:fb="http://www.facebook.com/2008/fbml">