I used one of those old 1960's posters that the NSA released to the public a couple of years ago in this one. This is a somewhat atypical source, but as I understand it, "Section 105 of the U.S. Copyright Act explicitly states that works created by the U.S. government are not eligible for copyright protection". And I think this would fall under the "fair use" clause if nothing else anyway.
I used one of those old 1960's posters that the NSA released to the public a couple of years ago in this one. This is a somewhat atypical source, but as I understand it, "Section 105 of the U.S. Copyright Act explicitly states that works created by the U.S. government are not eligible for copyright protection". And I think this would fall under the "fair use" clause if nothing else anyway.