Whenever you share a hyperlink on facebook it'll instantly find images online and at random picks one like a preview. How will you influence the preview image? Whenever a person shares the web site link on his facebook?
1. Range from the open graph and fbml xml namespace extensions for your html declaration
<html xmlns="http://world wide web.w3.org/1999/xhtml" xmlns:og="http://ogp.me/ns#" xmlns:facebook="https://world wide web.facebook.com/2008/fbml">
2. Within your
<head></mind> make use of the following meta tag to define the look you need to use
<meta property="og:image" content="your_image_here" />
Find out more about open graph protocol here.