When you import RGB like this, texturetweaker will keep whatever alpha channel was there in the old image, if it had one. If you don't want that, also Import Alpha (which can also be a black and white png), or Delete Alpha.

I wrote a start to finish object creation tutorial and in the Texture Tweaker related step there's illustrated instructions for how to use the tool to make .dds:

http://dino.drealm.info/den/denforu...php?topic=856.0

at post 8.