Thumbnails are small version of pictures, many to a page, which are used for reference in a larger collection. You can normally click on a thumb nail and it will load the full sized version.
BTW it is perhaps not a wise thing to post your email address here. Nice as we all are, you never know if there are any weirdos lurking about. If anyone wants to get in touch, it is best to click on the little envelope at the side of the person's name.