Rowan,
Generally we would prefer that only images that you have photographed yourself are used as reference. This is for two reasons really. First so that there are no copyright issues. Second because there are literally tens of thousands of older period reference photos out there and we just don't have room to host all the images that would come flooding in.
On another note I am really foggy about "who" really owns the copyright to various period images used in most of the reference books available. For example images taken by the USCE or any images taken by US personell hired as combat photographers are property of the US government and considered public domain (by most things I have read). The same might be considered true of German propaganda photos. Of course most copyrights have expired on WWII period photos which is why we see so many "bargain books" at places like Barnes and Noble with WWII era photographs.
Something tells me that a few publishers will disagree with my opinions above. But that is why we have lawyers I guess. They always tell us they can win a case no matter which side of the argument they are on.
Cheers,
Jim
