Just drag the image from the costumes tab to the sprite you want it on
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You could try right-clicking on the costume's picture in the "costumes" tab, and exporting it. Then, try opening the costume you want to paste it into in the paint editor, and selecting "import"
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Ok, so save one of the costumes on desktop etc., then click on edit the other costume, and then click import. Go to desktop, and click the first costume. It will import on top.
Hope it helps
-Hardmath123
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