Please check the below tutorials and see if it helps.
I read/viewed the attachments but all seem to make reference to Image Wrap covers only. No mention of solving Basil's issue with a Dust Jacket cover. I suggested he contact Blurb for any help they can provide.
Blurb's response has been:
It sounds like your best bet is to contact Lightroom support directly from the link below. They should have more detailed information to help you with your question (Blurb only handles the actual printing of your file or the conversion into a Blurb ebook).
(Click the appropriate category then look for a big blue button that reads "Still need help? Contact us").
I'd be interested in hearing what you find out.
I am unable to email support at Adobe and instead must phone, chat or post here. Leaving town for a vacation so I am hoping that Adobe will follow up from here.
Your 'Large' panel' 'small panel' are only text fields- a photo not permitted in these cells. Any photo will become like background!
Try all the different dust jacket templates-
A [+] sign on a grey background indicates a photo can be placed here. Otherwise only text allowed.
and another photo covering back jacket page AND the inside back flap. Text can overlay the photos.
You could get creative in Photoshop and make a collage image to fill front jacket + inside front flap as in the second template above.