A project's thumbnail is the display icon which represents a project on the Scratch website and can provide an attractive appearance for a project. When exploring the website for projects, the image of the project is the thumbnail. Thumbnails also refer to icons used to represent an object of some sort in other situations.
In the Paint Editor, costumes also have thumbnails on the left-hand side to represent them when being selected. All sounds in Scratch have the same thumbnail, that of a speaker and sound physically coming out of it. The thumbnails of all media in Scratch represent the media in a visual way.
Changing the Thumbnail
To change the thumbnail of your project, you must first open it up in the project editor. The exact screenshot of the stage when the project last saves will become the new thumbnail. Therefore, if the stage is blank when last saving and closing the project editor, the project thumbnail will appear blank when people browse the website and that specific project is shown.
If you do not want to take a screen shot of the project, but design a thumbnail for it, you have show the thumbnail when saving the project and then hide it when the green flag is pressed, to let any person see your project and not just the thumbnail.
To add a thumbnail quickly, you can use this script on the thumbnail sprite (be sure that is also at X: 0 and Y: 0 for best results)
when [t v] key pressed if <(username) = [Scratcher]> then // Replace "Scratcher" with your username. show end when [y v] key pressed if <(username) = [Scratcher]> then // Replace "Scratcher" with your username again. hide end
Currently, there have been glitches where the project thumbnail will only change for certain parts of the website. My Stuff often does not include the updated thumbnails, whereas the front page does. However, this may not be consistent among all devices and browsers. After changing a thumbnail, the project make take a while to update to where you can see the thumbnail on the creator's profile page.