Quote from StormFXI, personally, recommend Xnview. It has a full-fledged browser with BLP support, so you can click a folder and see thumbnails of all of the BLPs. I use it to export to PNG for editing, because it does an EXCELLENT job at preserving transparency. Then I use Photoshop/GiMP to convert to TGA (There's no reason to convert to BLP).
You can use either. BLP is a TGA, just with some different header info, IIRC. As I said above, there's no reason to bother with converting to BLP, as the game supports both.
^^ Ty tons for that. It'll be nice not worrying about conversion for homemade in-game icons.
Edit: Xnview is MUCH better than Irfanview.