Facebook introduced Open Graph in 2010. It promotes integration between Facebook and other websites by allowing them to become rich “graph” objects with the same functionality as other Facebook objects.
If you want to display specific image when you post your website link to Facebook or if at all if no image is displayed, just insert below meta tag between <head> section of your index page.
<meta property="og:image" content="http://yourdomain.com/images/filename.jpg" / >
You can follow the same process for 'title' and 'description', as show below.
<meta property="og:title" content="title here" / > <meta property="og:description" content="description here" / >