Translucent data is not stored in the alpha of a .png, just the transparent. Your backgrounds must match, for the translucent parts to look right. Black on black is the easiest, but if you use black behind your flash and don't have a black background in your sceen, your flash will have a noticable black edge.
If you want complex translucents data, you will need to add the effect to your sceen.
There may be another way around this in paint, that I am unaware of.