New Old Stock is a collection of vintage photos from the public archives which can be easily downloaded and are mostly free of copyright restrictions. Most of these pictures are available in the public domain based on the institutions participation in the Flickr Commons and the rules of Flickr Commons.
All photos are at the very least available for personal and non-commercial use (Medium articles, blog posts, personal projects, hero image for your 404 page). If you are curious of whether or not you can use it, check the institution’s rights statement through the link provided to the original Flickr posting. Most prefer that you link back to the original flickr photo and mention the institution by name.