Yes, this is a silly problem, from the plugins FAQ:
Some books are blank, just the “G” logo?
Due to licensing, sadly some image covers are not available through Goodreads API. If a books image is blank, you can define it in the settings page under “Exceptions” with your own cover.
To get the ID of the book, hover the cover of the book with your cursor on your website.
So basically, GoodReads won’t show some book covers outside of their own site.
I built a work-around in the plugin, so users can add their own, but it’s not user friendly right now. I will be revisiting this plugin shortly to make the process much easier.
Wow!. Lightening reply – i just posted. World is still a better place because of selfless people like you. Thanks a bunch.
Cheers. I’ll post back on here when there’s an update.