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You mean like the flash animation posted here?
Just take a screenshot of your desktop, and start modifying it.
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What you need to do is find the folder where all your desktop screensavers are saved. Then write down the folder location so you can remember it. Go to the folder where all your Scratch projects are. Open the file. Then scroll down the list of computer files to the Screensavers folder. Drag the icon with the name of your project underneath over to the Screensavers folder, then let go of the mouse. The screen will flicker, and then, when you open the Screensavers folder, your project should be there! However, it depends what type of file the screensavers in the Screensavers folder are. Both the screensavers files and the Scratch project files have to be the same type, or the screensaver of your project might not work. If you want to still keep your project saved into Scratch though, you will have to copy the folder, then move it. Hope this helps!
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