Hi Medieval Engineers.
I've noticed that some don't have a cover thumbnail for their items.

Here is the SOLUTION :

1 - Do a manual save before exiting and then click "no" when it asks if you want to save before leaving this way you will see your cover thumbnail.

2 - You can modify your cover but you have to publish your world again and don't save it before publishing cause you will erase your thumbnail again.

Where you can modify:

C:\Users\(your name)\AppData\Roaming\MedievalEngineers\Saves