Ok, I didn't do much, I basically used photoshop to boost the master levels... (how much lightness, darkness, contrast, and colors it has)
For the most part the only real problems I saw were the 2x scaling, which you may have done on purpose, so it is void, and the fact that you used RGB 000 black... As a graphics enthusiast, I can tell you that that is a no-no. Some programs substitute RGB 000 as transparent, (since it's technically reading data equalling 0. Thinking it has no information at all it treats it as trans.)
My suggestions, Use a very very dark color, like RGB 5,0,0 instead of black (note 5,0,0 is slightly red, and works well with red, dark yellow and dark brown.) For a "cool" black, I suggest 1,0,1 or 0,0,1 which will be slightly blue, and though not obvious, it can be interpereted by the brain will change the attitude of the sprite very slightly.
Granted, this still may not be what you're after, but I think it looks decent with a little more deviation in the colors like my version...