also, are you using .png image format for the alpha? Cycles doesn't fully support .png image (dunno why) if you use .png, you might get faulty result. convert the image to any other format that is not .png, like .dds, .tga etc.
ohh its a completely different setup youre using xD have you tried the one i posted? use mix shader instead of add shader. set the upper node to transparent. lower is diffuse + image. then link the image alpha to the mix shader factor ^^