Google Tests Related Dropdown UI on Mobile Image Results
https://twitter.comThis looks like a new feature that Goole might be testing on image results. Some users can see a drop-down with what looks like related search queries along with a thumbnail, from what it looks like they can appear from non-related sites to the main query image. In the example, the main image comes from Pinterest and one of the related dropdowns appears from Wikipedia.
This snippet can be a valuable source when you are researching a topic, keep an eye out for it in mobile image SERPs. I was NOT able to trigger it from AU and USA.