I found it. I have the exact same issue, and it's now fixed.

All the images that you will be using must be declared in the `images` section of your `__manifest__.py`.  Not only the banner and icon, but also any other screenshots that you mean to display in the `Description` section.

```

"images": ['static/images/banner.png', 'static/description/icon.png', 'static/images/banner.png', 'static/images/thubmnail1.png'],

```

Thanks for your answer @Piotr,

By "refresh your repository" I guess you mean perform a commit on the repository ? If so I did a bunch of them and it is not being re scanned by the Odoo agent. Moreover, The error/comment message on my module tend to suggest that I can trigger the re-scan whenever while I have noticed the daily scan actually runs once every day around 3 AM CET.

[UPDATE] Finally got to understand what you guys meant bu "re-scan". For others who are too slow to get it like I am , re-scan means going in your apps dashborad ( here: https://apps.odoo.com/apps/dashboard ) , selecting  "Repositories" in the left menu, and clicking on the "scan" option right next to the repo you want to re-scan. By doing this I managed to reach a 4/5 score as the "This module has no cover image (thumbnail)" error message remains a mystery.