This issue is usually related to metadata and how some crawlers scrape and collect information. Facebook Developer tools have a scrape tool that will allow you to recollect information for a page you have that isn't showing the correct thumbnail(s).
1. Navigate to this Facebook URL: https://developers.facebook.com/tools/debug/og/object/
2. Paste the URL of your liveBooks page that has the issue (i.e. www.yourdomain.com/portraits)
3. Select Show Exisiting Scrape Information first. You can review what info the debugger currently is showing.
4. Now select Fetch New Scrape Information and the tool will collect new information about your page. You may see a new selection of thumbnails for your post.
5. Test your URL by going to Facebook, pasting the URL and seeing what thumbnails show up for your page