Thank you for this link, Peter.
Before using this solution to get rid of the annoying Fnac Logo replacing the book cover during sleep, I decided to try to reconfigure an older Kobo (Aura HD) by doing a factory reset and setting up the Kobo without an account so as not to connect to a Fnac account or indirectly to the Fnac through a Kobo account.
To do this I followed the tips given here:
https://www.yingtongli.me/blog/2018/...kobo-rego.html
Despite the challenges for me, with virtually no computer savvy, I managed to carry out the operation!

So the Kobo was set up without giving any email or setting up any account.
I then added a book using Calibre and, after removing the Kobo from the computer, closed the sleep cover. When I opened it, lo and behold!, there was the dreaded Fnac logo instead of the book cover.

I certainly can't say that my manipulations were without fault but I do believe that I did follow the instructions. I'm giving this information to help others who might be tempted to try this solution!
I'll continue looking for a solution before I adopt the screensaver method as I was very pleased to have the book cover display on my Kobo!