Google Search to Include Thumbnail Images?

So I was doing a random search on our site and noticed that Google now began including thumbnail images in custom search results. An example:

Google search results with thumbnail images

I tried a similar search on Google’s main search engine, but the thumbnails didn’t show, so I’m guessing it’s something they’re experimenting on custom search and may or may not bring to their main search engine in the future.

At first I thought Google had some deeper integration with WordPress, since the results only seemed to show thumbnails for my blog results. But I’ve noticed a small amount of pages on our main site also had a thumbnail images (as the 1st search result above shows).

As you probably noticed, some of those images are obviously wrong and the purpose of this post is to see if anyone knows what Google is keying off of or if we can provide better guidance as to what the thumbnail image should be. The rest of the internet seems to refer to the image_src link tag for thumbnail guidance, but Google doesn’t appear to be using that.

<link rel="image_src" href="" />

If anyone has any idea, I’m all ears!

Update: Found the answer: Custom Search Help: Specify thumbnails