the easiest way is to modify your code in the index.html and upload a preview pic on to your server.
just paste the code below into your index.html right before closing </head> tag, fill in the appropriate values and then put your link through the fb link debugger to clear fb link cache so it will show the new preview image on fb -
https://developers.facebook.com/tools/debug
<meta property="og:image" content="PREVIEW IMAGE URL" />
<meta property="og:site_name" content="SITE TITLE" />
<meta property="og:title" content="PAGE TITLE" />
<meta property="og:description" content="DESCRIPTION" />
Willy