Paint.net works perfectly.
I tested the textures of the Sinope on TGA first, then I converted to .dds in Gimp. Horrors abound.
After a conversation with The_E, it was discovered that either I didn't understand how Gimp's plugin works, or that it didn't work at all, but when it was saved in .dds from Paint.net, the results were exactly the same as the TGA versions. So I'm sticking with Paint.net now, plus it loads up a hell of a lot faster, and it takes fewer mouse clicks to get what I want from it.
Oh, and I _was_ talking about opacity. An Alpha channel is the color channel that controls an image's Opacity.
100% Black = 100% Transparent
100%White = 100% Opaque
Thats basic graphic design knowledge, I'd pity anyone who attempts to create textures or even web graphics and not know that.