I am pretty sure I discovered what the problem was (is). This is what I think now. The print paper will not lie completely flat even in its frame with the glass front and heavy cardboard backing. So there are waves in the paper as it were, and flat spots--like a series of non-breaking waves on the sea. The light coming off the tops of the waves is different, I believe, from that coming off the flat areas. It's very subtle and often hard to see unless you hold the picture just so. Further evidence is that the photo, placed flat down on a glass tabletop, will not lie flat. Perhaps the wave portions of the pictures somehow got stretched a little causing that area to bulge.

I guess lankan_man has the answer but it's a lot of slow careful work.