Add new comment

The GIMP can edit PSD's directly, and the alpha channel is like a separate picture (as an image layer). In the Channels dialog (see volkan's picture), click the eye icons of the other layers to hide them, highlight the alpha channel layer (so it is the active layer for any changes you make), then work away as you would with any picture.

Edit:
And there are tons of GIMP tutorials around on the 'net if you look for them... Google is your friend.