The screenshot below says / shows it all — better than words. Thumbnails are unaffected and the original pics remain uncorrupted. Advance and reverse in this display mode of Photo Gallery [Edit > File View] sometimes momentarily removes the chess board blocks. By the way, these are always in the same place, top left, but not always the same number of rows and columns in various pics.

I have attempt a Repair, and indeed an uninstall then reinstall. The corruption remains.

Just so you know: in other Pic Viewers, for example IrfanView, this corruption is not evident in the same set of pics from the same folder; Windows native File Explorer displays the thumbnails flawlessly, as Windows Photo Viewer does the files themselves.

Suggestions gratefully received!