With Gimp you can create an image, set the background to transparent (advanced options when creating).
When you save (export) as .tga , use the defaults (compression etc.)
It's pretty painless.
I use Xara, Gimp, even Aialis Icon Workshop mostly for creating images, save as .png and convert to .blp using blpingconverter.
I am no artist.
ps. Paint can save as .png
