Unfortunately the cover images are blocked by AdBlock and are therefore not displayed.
In reality I finally understood that it is the name of the folders where the image covers are stored which is the cause.
To fix this it would suffice to rename the following folders :
by this (for example)
This is logical, the words advertisements and ads obviously being detectable by AdBlock.
Especially since in the Backend, the cover images are also blocked.
I specify that with AdBlock or UdBlock the problem is present. Mozilla or Chrome.