The last couple of years have seen a marked increase in the number of easily available, free, high-quality stock photos. My favorite of these sites right now is

http://www.sitebuilderreport.com/stock-up

-- which is basically a scraper of other stock sites attached to a search engine.

Note that many "free" photos have CC licenses, so do your homework and make sure you follow any attribution rules. Looks like the OP's link has photos that are completely free of attribution requirements.

The about page says these photos are curated from the web and submitted by users. How do you ensure that they fall under the Creative Commons CC0 license?

We only curate photos from CC0 resources. For both curated and user submitted images, we verify the identity of each author and perform reverse image lookups for each photo to ensure they're not copyrighted.