Major website issue. It seems that when you add a WordPress plugin to your site to handle a feature you’d like to use, you need to be very careful and make a backup of the site first.
I was experiencing problems with pictures in WordPress’ media library disappearing, even though a manual check via FTP shows that they do indeed exist. Since WordPress uses a SQL database to keep track of everything, and it appeared that the database had become corrupt, there’s not much to do except throw in the towel and rebuild the website. Maybe in the future I will spelunk into the details of WordPress database schema in case I need to fix up things. Right now, that just too much. For now, I’m going to get the web site into shape and them lock down some of the implementation.