Open your _D texture in GIMP.
In the layers dialog tab
Right click and choose Add Alpha on the layer.
Select the areas that you want to glow on the diffuse texture.
Right click and choose make mask on the layer.
Another window will come up.
Choose From Selection. Notice there is an inverse button there if the mask is opposite of what your need.
In the layers tab again right click and choose apply mask.
Go to the Channels dialog tab
Look at the alpha channel.
It should be all black except where you want the diffuse texture to glow.
If all is correct then you can then export it to the texturename_i.dds file with DXT1 compression.
There are several ways to mask when making them, so you may need to do it a few different ways.
As long as the results come up with all black except what you want to glow in the alpha channel (being white), then you are good to go.
Hope that helps more.
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