In the U.S. work was only protected from the time it was published, not created. Which lead to all sorts of problems in enforcing copyrights internationally. It's why Gilbert and Sullivan had to arrange to debut Pirates of Penzance in New York instead of England - to secure copyright in the U.S. before the work could be "pirated" by imitators.
For full information about U.S. copyright terms, here's some official documentation:
http://www.copyright.gov/circs/circ15a.pdf
When V.M. took many of her photos, they would have needed to have been registered to be protected. But so far as I know (and I may be wrong) none of her photos were ever published anywhere during that era. The changes in U.S. copyright law in the 1970s offered some retroactive protection to unregistered works. And the current law is all works are protected from the date of their creation, up until 70 years after the author's death.
But works which were published but not registered in the era when registration was necessary can fall into the public domain, as is the case of movies like the original
Night of the Living Dead or the American release of
Atoll K.