I'm wondering if this plugin exists. When you post a link to facebook, facebook scrapes the page for an image to use. On my blog, http://jessebrede.com, facebook never seems to pick up an image. For my main websites, http://www.blastro.com, I added a meta tag in the header that tells facebook what image to use. The format is:
<link rel="image_src" href="http://imagecache.blastro.com/images/320/jayzyoungforever.jpg" />
I'm sure it would be a simple plugin to build if its not built already.