Thanks very much for the clear explanations, Ken. I'll be working through your suggestions to see if they help.
I'm quite surprised that epub can be opened with my 7zip. Here's the innards of one of my epubs:
images\
css\
feedbooks.xml
about.xml
cover.xml
fb.ncx
fb.opf
footnotes.xml
main0.xml
main1.xml
similar.xml
title.xml
After reducing the resolution of the jpeg images, how do I put them back as a single epub file with Calibre?
Thanks.