in case you don't know how to put in an alpha channel (I assuming you don't otherwise you would have done it already)
Go into your painting program (needs to be able to handle layers)
Click Layer>Transparency>Add Alpha Channel
Then find your rubber tool, set it as big as possible, then set the opacity to 95% or whatever, and start colouring, be careful not to let go of the mouse button though otherwise you'll have to start again. There's probably a simpler way to do it, but it works for me.
Don't forget to save as a transparent png, with no colour values for transparent pixels, or any other format that handles transparency (I find png works best though)
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