[Globu]

In the Drowned image, I see faint stray pixels around the sprite -- be sure to use, say, the magic wand tool to find and remove those (if not wanted).

As for the angel one (and possibly the drowned one too -- I didn't check it for this), the problem is that you've got a dark "halo" of nearly-black pixels around it. Most of these should be deleted, but some require a judgment call -- if removing them destroys a certain element of detail that is good, then use the H/S/L or Brightness/Contrast tools to lighten them (and possibly increase color saturation) a bit.

Edge pixels under a certain threshold of lightness will show up in-game darker than you'd expect from looking at it in an editor. Unless you specifically want an area quite dark, you need to have the edges fairly light -- maybe just a bit darker than the main color. It took a bit of dinking around for me to get a sense of it, but you can reduce a lot of the pixel-by-pixel work by using the Magic Wand tool (Fuzzy Select in GIMP) with a tolerance (threshold in GIMP) of, say, 5 or 10. Experiment to get a good result.

Attached image should show you what I mean.