The Commons was started as a project between the Library of Congress and Flickr to increase access to archival photographs. Since it has spread to other historical or heritage institutions and has created a large pool of public domain, historical photographs.
Creative Commons Licenses allow creators to set different attribution, commercial, derivative and sharing restrictions to their work, which often makes reusing or sharing content easier than traditional copyright, while maintaining authorship. In an advanced Flickr search you can limit your results to display only CC licensed works.
Wikimedia has over 15 million photos, videos and sounds that are all public domain or freely licensed for use. Like the other Wikimedia projects, the content is user provided and curated.
Once you find an image you would like to use, choose the correct version of the file here and you're all set! Make sure to cite it appropriately, even though you can use it for free, you still need to give credit.