Okay, brain scratcher happening.
I bought the developer license, so have MyBookTable installed on 3 different sites. All 3 are updated to latest WordPress(4.4.1), and I just did the MyBookTable update for a 2nd site.
Same issue occurred. The plugin version is showing Version 2.2.1 on both sites (the first site I updated the plugin on, as of seeing your response and checking for the fix update, and the current site that’s now having the display issue).
4 out of 6 “larger” cover images missing, but their thumbnails continue to display fine.
Current site with the issue: https://discordjones.com/the-books/