In August, we caught Google testing showing image thumbnails in mobile search results. Well, it seems to be rolling out to everyone now - which is honestly surprising to me.
I personally can replicate it and it seems to mostly come up for product based search results. So maybe it has to do with product schema/markup and the rich snippets Google shows for them. Personally, I dislike showing a picture of the product in the search results when it is not in Google Shopping's user interface but it seems Google is going ahead with this user interface.
Here are some screen shots from my iPhone 6S (soon to be 7):
What do you think of the product images in the mobile results?
Update: Structured data is not required for images to show up.