Wikimedia Commons is an online repository of free-use media (images, sounds, and other file types) and, like Wikipedia, is hosted by the Wikimedia Foundation. Files hosted at Commons can be used at any Wikimedia project or downloaded for external use.
There's so much in there it can be daunting to try finding it all, but the search bar is definitely your friend. Know that all photos (and other files, but photos are by far the most common). Commons been around since 2004, and in 2013 notched its 100 millionth edit. As of early 2017, the Commons hosts more than 35 million files (and see more impressive statistics here).
Many archives, libraries and other institutions have donated images to Commons, and other images come from the public domain. And then a fair number also are simply uploaded by Wikipedia editors back from vacation.