I use paint shop pro and the tutorial pic for making translucent hud's. Im not familiar with Gimp or photoshop, but I'm sure they has at least similar features. I'll explain how I do it in PSP and maybe you can apply it to your own programs.
1.open pic in PSP making sure it's on a background that is rgb000.
2.right click pic in layer tabs, create new mask, from image.
3. when window pops up choose -any non zero source value-
4.On the top bar, Selections, from mask
5.Once again,On the top bar,Selections, then Save Selection to Alpha channel.
6.Erase or merge mask layer with pic layer.
7.save as .tga
Hope that helped.