Ben, that’s not true. a TGA needs a proper alpha channel. black only works if you choose ‘add’, and in that case, the image does not have to be a TGA.
Transparent areas of an image show up as a checkerboard pattern in the image window, this is true for both GIMP and PhotoShop.
I’m more familiar GIMP than PhotoShop, but the way I make a transparent texture in PhotoShop is to choose ‘transparent’ for the background color when creating a new image. Then the alpha chanel is already created and I just paint normally.
In GIMP, you can do it that way, or right click in the image window and choose layers->add alpha channel. Then to get chroma-key like transparency, right click in the image window and choose filters->colors->color to alpha… set the color you want to make transparent, and click OK. Then save as TGA.
For you’re flares ndnchief, I would save the image as JPEG, and set the paint mode to add. This will get a similar effect to an alpha channel without all the trouble. JPEGs can also be compressed to a much smaller file size, so your game will run faster.