Seriously: it depends on what niche you are into.
Go through them one by one, some of them have a lot of "people pictures", some have a lot of "business pictures", some have "government pictures". Pick the sites which suit your purpose.
Flickr.com has an option to search for public use photos that can be redistributed for commercial use (and they're not protected). I assume you can use them ,and there's millions on there.