This was the issue that I usually faced while checking comparative images at efi google site. All image links (of google group images) will not show the thumbnails. It may be that my computer was sending requests to Google server to show thumbnails of a large set of images at genus or family pages. Sometimes, it was resolved by verifying that I am not a robot. But sometimes it did not.
Fortunately, this problem seems to be over in our new site as images are mostly stored in our own site and server.