Found one of my pics on http://animalia-life.club being offered as a free download.
It's not like they don't know, it's actually got iStock as part of the URI! (Unwatermarked).

You can't search the site as such, you go onto pages via links and at the bottom of each page are some photos to freely download. Getting from page to page is random, I found my file via a Google reverse search. I'm imagining a fair number of these images are stock files.

I have reported my file to iStock. (not holding my breath).
Gotta luvvit: on the first page of that site, they say: "Any reproduction or publication of information from our website without permission - is prohibited".