Google is now showing randomly selected thumbnail images left to the results. Google crawls the web to find new or updated web pages, but these pages can be grouped in different categories. For example, some web pages include videos. If Google decides that a site contains a lot of videos, it might add it to Google Video and show thumbnails next to the snippet.
The query Amir Blumenfeld shows thumbnails next to a blog that occasionally posts videos and next to a "tumbleblog" that mostly consists of videos (the first and the seventh result from the screenshot below). The videos are hosted by other web sites (in this case, Vimeo) and the thumbnails are only included for homepages.