Unsplash [1] is a nice alternative that I’ve checked out a few times. Use for any purpose is free. No attribution required (though it’s recommended). It has proper documentation on the license, privacy policy, etc. There are even apps available to browse, download images from the site and use. Anybody can contribute images to it.
P.S.: I’m not associated in any way, except as an end user, with Unsplash or any other photo/image service.
[1]: https://unsplash.com
