User blog comment:CitizenA/Paint.NET silhouette plugin/@comment-24.184.105.92-20130218160257/@comment-5906821-20130218211037

I'm not sure what the benefit of it is, to be honest I've never tried simple transparent .png's, but it's how the official sprites are formatted and most of the scenarios I looked through happened to use it. But if transparent.png's work just as well, then yeah, that might be a quicker way to use custom images.