Amusingly, in my attempt to flee the Fnac Logo, I set up an account with Walmart. On success, the Kobo asked me to agree to the Walmart conditions and also to the Fnac conditions !
I had not mentioned Fnac in the setting up process.
Of course, I got the Fnac logo on sleep!
The affiliation was to Fnac. I changed this manually to Walmart and got the bookcover on sleeping.
I did a synchronization and the new settings menu included "show book cover". So I assume this is the Walmart firmware and not Fnac.
I don't know how stable this is. However it may be a solution for those who have the Fnac logo problem and who wish to have the book cover instead.
I will come back and give more information if the Walmart solution proves unstable and I end up with the Fnac logo again!
Until then, I hope this will be useful to other users!