Unsplash – Beautiful photos free to use under the Unsplash License
Unsplash is a really cool resource.  We actually use it (paired with another 'sister service' called Unsplash.it) to provide ever changing and semi interesting 404 error pages for our web app... I blogged about how we do it a while back - http://devan.blaze.com.au/blog/2015/11/3/errors-dont-have-to...