Our iOS app has the option to search Flickr for stock images on the "create a post" screen. We filter the Flickr feature to only show photos with creative commons license zero: http://creativecommons.org/about/cc0
Flickr has a lot of different license options that users can set but we only pull ones with that creative commons license zero. How that is described is that the copyright holder waives the interest in your work and place the work as completely as possible in the public domain so others may freely exploit and use the work without restriction under copyright or database law. What this means is that we are only pulling photos that the Flickr user has uploaded with no restrictions, free to share. However, we don't know if Flickr does any check to verify their copyright.