Facebook uses og:tags and the Open Graph Protocol to decipher what information to display when previewing your URL in a share dialog or in a news feed on facebook.

The og:tags contain information such as :

1. The title of the page
2. The type of page
3. The URL
4. The websites name
5. A description of the page
6. Facebook user_id’s of administrators of the page ( on facebook )

To Show particular image as thumbnail while share the URL on Facebook, use this following meta tag:

<meta property=”og:image” content=”http://www.example.com/images/image.jpg”/>

Note: The thumbnail image size must be in 200 * 200 Size

Once you have implemented the correct markup of the og:tags and set their values, you can test how facebook will view your URL by using the Facebook Debugger

Some Facebook og meta Tags are:

<meta property=”og:site_name” content=”xxx”/>

<meta property=”og:title” content=”xxxx”/>

<meta property=”og:type” content=”xxx”/>

<meta property=”og:description” content=”xxx” />

<meta property=”og:url” content=”http://www.example.com”/>

<meta property=”og:image” content=”http://www.example.com/images/image.jpg”/>