I display hero images across the top of my blog pages. However, I use CSS images rendering through div tags and I read that google doesn't typically index those. See example [French Love Words][1] page. 

So the images aren't included as thumbnails in SERPs, see below.

Can you please confirm if there is a way to direct google to index the images like with an image sitemap?

Does an image have to be included in the IMG tag to be considered for a thumbnail in the SERP? 

[![enter image description here][2]][2]


  [1]: https://vidalingua.com/blog/french-love-words-sayings
  [2]: https://i.sstatic.net/Crluh.png