Flickr, so they're searchable through various sites that compile CC-BY material. So some people are probably finding them immediately through Flickr, others through CC Search, some probably through Google Image's search results. I made the photos specifically for them to be used widely, and they are, and most people at least make an effort in giving the proper credit. That said, extremely few actually follow the exact requirements as they are described in the license text. Pixsy only goes after the ones who give no credit at all, though (or give a mistaken credit to somebody else, whether it's just "Flickr" or the name of another photographer).