hmm, i never did have any succes when doing it like that, try adding an alpha channel, now if u were anything like me, my first question is, wtf is an alpha channel!?

basically what you do is create your images in a program that supports .TGA, then you make an alpa channel, what an alpha channel does is that, anything black will be cut off...

here's an example, the black and white image is the alpha channel:

http://help.adobe.com/en_US/AfterEffects/9.0/images/str_channels.png

good luck, hope that helped

-dark out