IIRC there's existing case law in copyright law about the amount of creative work required to perform "transformative work" to an existing work. I figure that essentially, if you barely touched up the photos, you'd not have "transformed" the original work, and it would remain in the public domain, but if you performed sufficiently extensive restoration, it may count as a transformation.

(going off of memory here and a bit of speculation, hope I didn't misremember it)