The thumbnail is updated whenever you share or update a project. It is a picture of the stage at the time when you shared the project. It lags when updating, so sometimes it doesn't update for 3 days. You can control the thumbnail by arranging the sprites when you upload the project.
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If you'd like to have a cover page on your project, make one and set it so that it will disappear when the green flag is clicked. However, if you would like it to show a certain part of the project, then play the project and stop it at whichever part you want its thumbnail to be shown. Then, you share the project.
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