After extracting a lot of interesting information from web pages (dates, locations, author names), Google experiments with displaying the number of images embedded in a web page and a list of thumbnails above the snippet. Patrick Altoft posted a screenshot that shows a search result which includes 17 images, but only 9 of them were picked by Google.
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.