Looking for changing the thumbnail image while sharing your website or blog link  or Customizing the default thumbnail each time on Facebook?? You can do it by adding a simple meta tag in your page’s header section.

Your meta tag should look like this:

<meta property="og:image" content="http://http://www.example.com/images/xyz.png"/>

As per Facebook’s guidelines, your custom image should be more than 200x200px.

While sharing the link, Facebook grabs in information from GRAPH objects and display them beautifully. Apart from this, there are other information as well, which can be customized like

<meta property="fb:app_id" content="xxxx"/>
<meta property="og:site_name" content="xxxx"/>
<meta property="og:title" content="xxxx" />
<meta property="og:description" content="xxx" />
<meta property="og:type" content="xxx:photo">
<meta property="og:url" content="http://www.example.com/content/xxx"/>
<meta property="og:image" content="http://www.example.com/images/xxx.png"/>

Further, you can check how Facebook looks at your URL while sharing using debugger tool http://developers.facebook.com/tools/debug and tweak the things in order to make URL sharing more impressive.

Reference:

• Open Graph Protocol
•  Facebook Documentation