Can be turned off: http://googlesystem.blogspot.com/2013/08/changes-to-chromes-...
Although it only appears to be enabled on non-Linux platforms
That said, I spent the evening trying to figure out how to fix Firefox's new tab page (half the thumbnails aren't there because Firefox doesn't store them for https:// sites to protect my privacy) - if you go into the thumbnails subdirectory of the Firefox profile directory, the files there are named using the MD5 hash of the URL of the page - just put a PNG in of the appropriate size and you can customize the blank thumbnails to your liking.