Google is now showing image thumbnails for more mobile searches. Based on data collected from RankRanger, images are showing up for 44% of search results now (up from 15%).
This combined with Google's mobile-index and various other updates shows that mobile search is where many of Google's improvements are focused on.
JSON-LD for SEO includes any relevant images in your structured data automatically. It's not clear if Google is using structured data or crawl data to pick the images but in either case, you should make sure to have relevant and high-quality images available wherever you can.
SEO never stays still which is why it's important to use apps that stay up-to-date with it.
I create multiple updates to JSON-LD for SEO every month to improve its structured data creation. Once you install, you'll receive these updates for the lifetime of your store so you can be confident that it's always following the current best practices.
Eric Davis