Alpha transparency has outline

That link seems to be for experienced photoshop users, but ones who never worked in games - they use lots of layers naturally, and know layer-alpha, but have never through about end-result alpha. I'd guess your gimp images are alway just 1 layer, so most of that wouldn't apply anyway.

I've found the most common reason for an outline is a light colored image using black for the transparent pixels. For example, in gimp you start black, paint something light, and select "Color to Alpha". As it fades out over a few pixels, it also turns black-ish. The fix is, after you have alpha, paint a little outwards, using that same color. It always takes me 15 $$anonymous$$utes to remember how to do it, but it can be done (I think you lock the alpha layer first).