This question already has an answer here:
I've searched high and low and finally hit upon alpha channels, but I don't know how to configure an alpha channel to suit my needs. I'm trying take this image...
...And turn it into this. As you can see the top portion is 100% opaque, and the bottom is 0%. So its a kind of "alpha mask". How can I do this with Photoshop or Gimp or whatever, and save it into a 32-bit PNG?
I'm not simply trying to "cover" or "mask" my image like this question. I'm trying an alpha mask which is a whole different ballgame.