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: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.