Copying & pasting the PreMultipliedAlpha tag wouldn't make any difference. You could just check the box under the Image tab.
Also, loading a .png wouldn't make a difference, because SKS will convert into a .tga upon import.
PurrBall made me think of something, though;
Vamp, how did you create your .tga? Did you import a .png, save as .tga in Photoshop, use the frame builder...?