[lowfighter]

If there's a tgb than the game uses that image instead of the tga. If you extracted the map textures you'll notice that most of textures have both a tga and a tgb, and always the tgb is higher size (higher resolution) than the tga. I suspect the tga is used with lower lanscape detail setting. Now if there's only a tga and no tgb, the game will use that one. But I forgot to use the phrase "I believe" here, so the best is if you can test it yourself...