I had discovered how at one point, but I don't remember. You do have to edit the code though.
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You could also screen-cap it and edit it.
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To save costumes as .pngs, you can't use gradients and related effects, otherwise it is saved as a bitmap(.BMP). Hope this helps!

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ScratchDude101 wrote:
To save costumes as .pngs, you can't use gradients and related effects, otherwise it is saved as a bitmap(.BMP). Hope this helps!
That's GIF.
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Just save as costume, then change the costume from .bmp to .png.
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