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If you want a transparent background you can use image magick. For example

convert opaque-image.png original.png -compose CopyOpacity -composite PNG32:output.png

The original and opaque one need to have the same size though.  I'm using linux and wrote a tiny bash script for convenience.

I know this is a old post, but it's the first and only result on google on this question, so it might help others.