Save it as a .png or something. The alpha channel map needs to just be an image, the photoshop thing is if you are embedding transparency within an image, which you're not, you're creating an image to dictate the transparency of another one.im so sorry to be a pain but i cant figure out this alpha channel stuff, i have an alpha channel created in photoshop but every time i try to export using a .tga format the alpha channel tick box is greyed out seriously stressing over this now