As of 2015, some of the best sites to get high-resolution, high-quality stock photos include Unsplash, Little Visuals and IM Free. The photos on these sites can be used for commercial purposes, and the sites constantly release new content.
Unsplash and Little Visuals are both at the top at providing high-quality, free photo stock. Their images are released under Creative Common Public Domain licenses, allowing use of the images for any purpose. Both these sites offer subscription services that send images to an in-box for free. IM Free's offers a collection of curated images. Although some of their images might not be released under the Creative Common Public Licence, images are available for commercial purposes.