I haven't really looked at this topic in detail yet, but here is the basic deal:
- the bitmaps are additive blended, which means that black == transparent and white == opaque, so dark colors will blend more
- the glow bitmap is drawn on top of the sun bitmap, but at 50% opacity
So based on the images posted, it's doing exactly what it's supposed to do. The glow bitmap is just overpowering the sun bitmap. Your options are basically to remake the glow so that it merely adds to the sun bitmap instead of pretty much replacing it like now, or incorporating the sun bitmap into the glow bitmap so that you will still be able to see some sun bitmap details in the glow bitmap. It's easy enough to do in GIMP, I just did a color-to-alpha conversion on the black of the glow bitmap, then copied and pasted it as a layer over the sun bitmap, set it to screen and with 80% opacity. I haven't tried it in-game, but it should probably work pretty well, though you'll might have to tweak the settings a bit and try again if the values I used don't work as you like.
Also remember that although you have to specify a sun glow bitmap, no one said that it can't be a totally black image, and therefore fully transparent to the game.