Facebook like box widget
According to this tip, to use a thumbnail of your choice simply to include an additional <link> tag in the <head> portion of your html file that points to the URL of the appropriate thumbnail image as shown here.
<meta name="title" content="Digital Inspiration" /> <meta name="description" content="Technology and Software Blog" /> <link rel="image_src" href="http://www.labnol.org/screenshot.png" />
You may already have the <meta> description tag in your site template but make sure that you also add the <meta> title tag for blog thumbnails to work. The <meta> title can have the same value as the actual <title> tag of your web pages.