Thanks! This look familiar. You answered this question before, but I couldn't find the post. Shame on me. I have saved it to my "misc" directory with a cryptic filename, for next time.
For some reason, I still cannot exorcise the old thumbnails. This is an album that I created before I knew that I could disable the "square thumbnails" plugin. I actually created square versions of the images with white borders to fill the extra space! The images themselves do not have the borders--I think I went back in and replaced them with non-square versions after the thumbnails had been built. Gallery2 is stubbornly refusing to rebuild them from the non-square images.
I think I am going to have to delete and recreate this album.