Thumbnail preview for Google`s mobile search results

Quick question - do any of you know how to specify which image is used as a thumbnail preview for Google`s mobile search results? I`ve seen lots of speculation, but nothing definitive. Wondering if anyone has figured this out yet? I`ve tried to reverse-engineer what I`ve seen in the results, but the results aren`t consistent enough.

For anyone not sure what I am talking about, here is an article: https://www.seroundtable.com/google-mobile-image-thumbnails-search-22705.html