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Open the android.png in GIMP.
Select Fuzzy Select Tool.
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Click on the background of the icon.
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Click Select > Invert to select the foreground image.
Click Edit > Copy to copy the select image to clipboard.
Close the original file.
Click File > New to create a new image
Expend the Advanced Options in the Create a New Image dialog.
Select Transparency in the Fill with option, and click OK.
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Click Edit > Paste to paste the copied image as the foreground.
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Run the Android Application again, the background have been changed to transparency now.