If you have a web page then you share its link via WhatsApp but you did not see an image thumbnail of your page, you must be missing these meta tags on your page:
1: title
Maximum of 65 characters
<title>your keyword rich title of the website and/or webpage</title>
2: description
Maximum of 155 characters
<meta name="description" content="description of your website/webpage, make sure you use keywords!" />
3: og:title
Maximum 35 characters
<meta property="og:title" content="short title of your website/webpage" />
4: og:url
Full link to the current webpage address
<meta property="og:url" content="https://www.example.com/webpage/" />
5: og:description
Maximum 65 characters
<meta property="og:description" content="description of your website/webpage" />
6: og:image
Image(JPG or PNG) with a size less than 300KB and minimum dimensions of 300 x 200 *. This image should be served via a HTTPS connection with a valid non-self-signed certificate.**
<meta property="og:image" content="//cdn.example.com/uploads/images/webpage_300x200.png" />
I got this lesson from this forum page: https://stackoverflow.com/questions/19778620/provide-an-image-for-whatsapp-link-sharing