Google has confirmed with Search Engine Land that it is currently running an experiment to index images found within PDF documents and display those images within Google Image Search.
This feature was first noticed by Alex Chitu, who said he wasn’t sure if Google had the ability to find and display images within PDFs in their Google Image Search results. He was right, it was new.
Google told us they started testing this feature in June.
Here are some screen shots of image results within PDFs in Google Image Search.
As Alex says, you can find more examples of this by searching [google pdf site:static.googleusercontent.com].