As far as I know, I don’t really think there’s a way to publish a thumbnail with a MainModule. The best you can do is either leting people know what the model is about in the model description, or linking them to a YouTube video that showcases the model (if you’re willing to do it).
This method may sound very strange but it works. You can put a Sky and set SkyboxFt to the image you want. I’m not sure how to explain it. Basically, if a Skybox is inside of a model, sometimes it acts as the thumbnail. I got: from doing so. I would recommend grouping it and then doing it separately.