Google is once again testing image thumbnails in the search results snippets. Well, Google has done it for some queries and pushed them out but for standard snippets, Google doesn’t by default show image thumbnails for them.
For example, here is a snippet I see myself on mobile without the image thumbnail:

But here is a screen shot of the same page showing an image thumbnail in Google search from Jason Mun on Twitter:

Google is constantly testing these image thumbnails for many years.
Forum discussion Twitter.