Thumbnails are a powerful way to provide users with more information about the listings returned, improving both presentation and usability. Users of Google's custom search offering now have the ability to add thumnail images on search results automatically.
Many Adsense publishers, those that use the Custom Search Element, already provide thumbnails but have done so through the use of rich snippets. With its most recent layout modification, Google has made the life of a developer a bit easier. Custom Search will now crawl websites to find representative images and add them automatically to the search results snippets.
It was relatively easy to get thumbnails to appear in Google's CSE, but developers were forced to use PageMap data in the <head> section of an HTML page or use the thumbnail meta tags. Google's crawlers have obviously gotten a lot more sophisticated over the years and this announcement is definitely proof.