I'm not sure where it is in pantshop but in photoshop there is a channels tab next to the layers tab just click that and you get the RGB, R, G ,B and Alpha. it works just like a layer.

Now don't forget when you paint on the alpha it's in grey scale and the White areas show the texture and the Black is transparent, shaded of grey are partial transparency. your going o have to paint that black corner on the shield black on the alpha channel so you don't see it