It is an index of billions of images which the tineye can match images it has indexed previously. (my brain is getting tired)
When you paste a link to an image file to tineye, it will find matching images and show the URLs and matching images ...or in this case, parts thereof.
(PedroHinCome along & ride on a Flantastic Voyage,
Sun 3 Oct 2010, 5:26,
archived)