The feature is not available for everyone, but if it's launched, I suspect that Google will only show the enhanced snippet for pages that embed a lot of images.
A similar feature is available if you click on "search options" and select "images from the page", like in this search for [mouse]. Google will show at most 2 thumbnails next to each search result. Thumbnails "can help you quickly identify whether the page is relevant to your search term. For example, if you see an image of a furry little critter when you search for [mouse], you can probably deduce that the page isn't talking about computer equipment," explains Google's help center.
