Photoshop would be the best tool for editing TARGA (.tga). It enables you to edit the alpha channel and give your image some transparency.
If you create that channel you'll have to save your image into .tga 32bit channel (3 channels plus alpha; 8bit per channel). Then just open the tga with TexView and save it as .paa!
If you want an editable image sample (.psd format), just message me!
Its very simple to deal with...