When a person posts a link on Facebook, Facebook automatically finds a photo to be used as a thumbnail that shows up in the news feed. Is there some sort of (HTML) markup to sur
keep in mind that if everything doesn't work, than, may be, you should reload Facebook cache. http://developers.facebook.com/tools/lint/
What you want are the <meta>
tags that Facebook defines in their documentation on Facebook_Share/Specifying_Meta_Tags.
You don't put the metadata around the image itself, rather, you put it in the head of the document, like this:
<meta name="title" content="The Title Of The Link" />
<meta name="description" content="A description of the link would go here." />
<link rel="image_src" href="http://www.example.com/images/image.jpg" />
The accepted answer is now out of date. The new way to do it is with an open graph tag e.g.
<meta property="og:image" content="http://example.com/picture.jpg" />