If they are in that "Impact" magazine, I'd look at the original magazine to see who the photos were credited to. If they say something like "Army Signal Corps," I'd credit the magazine you got them from. If you can find the originals in the National Archives, then use the NA archival reference number. Of course, do all of that before your drop-dead date!

If I were in your situation, I'd just credit the magazine.
As Jack said, if it's a government work, it's in the public domain.
kevin