The easiest way is just a link tag:
<link rel="image_src" href="http://stackoverflow.com/images/logo.gif" />
But there are some other things you can add to your site to make it more Social media friendly:
Open Graph Tags
Open Graph tags are tags that you add to the <head> of your website to describe the entity your page represents, whether it is a band, restaurant, blog, or something else.
An Open Graph tag looks like this:
<meta property="og:tag name" content="tag value"/>
If you use Open Graph tags, the following six are required:
og:title - The title of the entity.og:type - The type of entity. You must select a type from the list of Open Graph types.og:image - The URL to an image that represents the entity. Images must be at least 50 pixels by 50 pixels. Square images work best, but you are allowed to use images up to three times as wide as they are tall.og:url - The canonical, permanent URL of the page representing the entity. When you use Open Graph tags, the Like button posts a link to the og:url instead of the URL in the Like button code.og:site_name - A human-readable name for your site, e.g., "IMDb".fb:admins or fb:app_id - A comma-separated list of either the Facebook IDs of page administrators or a Facebook Platform application ID. At a minimum, include only your own Facebook ID.
More information on Open Graph tags and details on Administering your page can be found on the Open Graph protocol documentation.
http://developers.facebook.com/docs/reference/plugins/like
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