[Fly!]

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[Tomthetom]

HAVE YOU TRIED TO COLOR IT TO ALPHA??

[Fly!]

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[Fincuan]

Layer -> Transparency -> Add Alpha Channel

After that select the area you want to make transparent on your "working" layer and delete the area.

[BomberBoy]

I also had problems with GIMP creating a TGA with Alpha channel that the game would recognize. (I was making new default skins) Finally I had to resort to saving the skin and the alpha as separate files and combining them into a TGA/Alpha using an old MSFS utility called DXTBmp.