Gimp is a fine paint program. A bit limited in its functionality, but it works for what you're doing. What you need is a converter for the TGAs. The real issue is that when you save a TGA without using the correct tool, it creates an all-white alpha channel, which means that everything is going to glow as bright as it can.
Go download
DXTBmp. It was created for use with the MS Flight Simulator series, but deals with TGA files perfectly. Once you have downloaded and installed, open up any texture you wish to edit using DXTBmp. You should see something similar to image #1 below.
On the righthand side near the top of the interface is a box showing a black-and-white image. This is your current alpha channel. For textures that someone else created, all you need to do is the following: click on the button below the alpha that looks like an arrow going into a folder. This will open up a dialog box similar to image #2. Navigate to an appropriate folder (I suggest something outside of the actual BC directory) and save as (NAMEOFTEXTURE)_alpha.
Now click "File", then "Save As", and then "24 bit BMP image" as in image #3. This will allow you to export a standard BMP that you can edit to your heart's content using Gimp.
I wouldn't recommend editing the alpha, since it only controls the glows, but if you want to add something to it, feel free.
Once you have edited things to your personal specs, go back to DXTBmp and we can reverse the process.
Open your edited BMP. Notice that the alpha channel section is pure white (#4). We need to correct this. Under the alpha window, click the button that looks like an arrow coming OUT of a folder. A warning will pop up (#5). Click OK and move on. This will open up a dialog similar to the first one you messed with. Navigate your way to your exported alpha channel from the first step and open it. You are now ready to export to TGA. Click on "File", "Save As", "Targa with Alpha" (#6) and save it with the exact same name as the original. This is important! If it isn't named the same, BC won't recognize it. Your texture is now ready.