How to edit skins?

[Rookie25]

I've edited heaps of liveries on rFactor & iRacing - simply by changing logos etc then resaving.

In AMS this doesn't work, the liveries are saved almost completely transparent, and when I edit them they don't show up in game. 6 months ago I worked out how to do it in AMS, and now I have completely forgotten - does anyone know?

I know it has something to do with the Alpha Channel, and quite possibly using a layer mask, but I can't quite get it happening. (using gimp)