Thank you Jorge for your reply. This is my extension on AMO. I followed these instructions of the Extension Workshop to upload each version of the extension, but I have never been able to add a screenshot .
OK, I managed to upload a screenshot. I was focusing on the “Add icon” button and didn’t notice the “Add screenshot” just below it , but I can’t swear it has been always there.
Thank you for your support @jorgev and sorry for my carelessness.