Flickr Commons contains a vast array of free-to-use public domain images from many collections, well tagged and annotated.
In general, I prefer Wikimedia Commons (http://commons.wikimedia.org) though it is slightly harder to find public domain images there (many are CC-licenced) and the consistency is not as good.
Comments
Because the license for any given Flickr photo can be changed by the owner at her or his discretion - e.g. from one kind of CC license to another, or from CC to full copyright - it's advisable to save a screenshot of the photo source page when you download a given image. A good papertrail is always a good resource (and defence, if it comes to it).
That said, I would hope that a Flickr space designated for CC and public domain pics might provide more reliable and consistent licensing.