Yup, "file too big." There are an enormous number of image files-- 761, to be precise.
I looked at the file with a browser after unzipping it and it looked fine, as I said. I chopped 5 megs out of it by deleting half the .html files and the associated image files, rezipped it and renamed it appropriately. The result was the same-- "file too big."
I think your suggestion that the number of images is the issue is correct. Though the task is beginning to look Sisyphean, I'll chop out another 50% of the .html files and their supporting images and try once more. If that doesn't result in something useful, I'll throw in the towel.