I honestly don't know the answer to that. I don't know how the reverse search engine does its magic. All I know is it works. I caught a car dealership in Georgia using 3 of my photos on his dealership website. He copied them right off one of my pages that contained a large copyright notice.
Nice!
From the tineye.com faq:
When you submit an image to be searched, TinEye creates a unique and compact digital signature or 'fingerprint' for it, then compares this fingerprint to every other image in our index to retrieve matches. TinEye can even find a partial fingerprint match.