Well, two ways to do this I know:
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Copy the original button to a new place (it will bring its bitmaps with) and then blank out all the various settings in the copied button - macro, tooltip text, etc. Then replace them with your own macro/tooltip, etc. That normally works, I do it often.
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Export the bitmap from the original to your desktop or elsewhere, then import the same into your new button. You might want to do this with all 3 bitmap size variants (16, 24, 32 px)