As an author, I have struggled with this one as well.
The US Government generously considers its own material, including photographs, as being "in the public domain", and charges nothing in the way of reproduction rights. if you buy photos from them, you can use them as you wish (except that you cannot, of course, claim your own copyright for them). However, their publications may include pictures which originate elsewhere - where they may still be copyrighted. Very confusing!
Matters are being made worse by the fact that, although the period of time that copyrights last varies internationally, the tendency seems to be for it to increase.
Tony Williams: Military gun and ammunition website and discussion forum