Using Public Domain Images

In August I wrote about how to cite  Creative Commons photos.  Today I'd like to investigate how or why to cite Public Domain photos.  The following is not to be taken as legal advice, but as general guidelines for academic work, in a school setting. Rights are country-specific, so there is no "one size fits all" discussion of this topic.  Wikipedia begins the page on Public Domain with this paragraph: "Works in the public domain are those whose exclusive intellectual property rights ...