Stock photo agencies.
Negative Space has free stock images that are released under the CCO license. The images can be used in personal...
Superfamous has super great textures inspired by nature. There is a set of high resolution images, that have been released...
For personal or commercial use, PhotoCollections.io has images available for download and published with the Creative Commons license.
Skitter Photo has a large collection of free images to use for personal and commercial use.
A good sized collection of stock images to choose from. The images are also allowed for commercial use.