Spark's Sprites

[Terminal Devastation]

Gradual transparency is best left for the game's engine to take care of more than the sprite itself, in my opinion, so I'd just leave the whole thing black.

I'm sure there's ways to create the illusion of gradual transparency just with the sprite, but the only one I can think of, dithering, is likely to be a worse option that what you already have. I'd need someone that knows the technique to chime in about that though.