Create an image file that you want as the custom thumbnail. It must be 256 by 256.
Save this file as a *.png file in the Bloom folder for the book with this name:
custom-thumbnail-256.png
Do the steps in Publish to Web to upload your book to Bloom Library.
Wait 10 to 15 minutes for the Bloom Harvester to update your book.
To find the Bloom folder for your book, go to the Collections tab.
Right-click and click Show in File Explorer.