The Ripl iOS app has the option to search Flickr for stock images when adding photos. We filter the Flickr feature to only show photos with creative commons license zero.
Flickr has lots of different license options that users can set. The Ripl app only pulls images with that creative commons license zero. How that is described is that the copyright holder waives the interest in your work and places 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 checks to verify their copyright.