I have some .png web images that I need to tweak. When I open them in my graphics app (I use GIMP), they show up as indexed, with no graded transparency (not sure if that's the correct term; I'm referring to the gradually increasing alpha value around the edges that makes the image look smooth to the eye, like font antialiasing). However, the alpha grading is there when I view the image on my site. Also, it has an alpha channel in my app, and when I change it from indexed mode to RGB, the grading appears.
That's all fine. The problem occurs when I tweak the image and re-save it. It's fine if I save in RGB mode, but the file size is 4 times larger than indexed mode, so I prefer the latter. But when I try to save it, I lose the alpha grading (even though the channel's still there). It's gone in web view and if I re-open the image and change back to RGB mode, it doesn't show up.
Is there some trick to saving a png in indexed mode while keeping the smooth image edges? (If it matters, the image file is a simple, one-color, icon sheet.)