transparency - Gradient fade a PNG image using Photoshop

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.

Answer

You can use clipping masks to achieve this effect!!

1. Create 2 layers in photoshop


2. In the 1st layer place your image content


3. Click the 2nd layer


4. Select White as the foreground & background color (by clicking the color swatches on the toolbar)


5. Click the gradient tool, and choose the "opaque to transparent" gradient (the 2nd gradient in the palette)

6. Draw a gradient on the 2nd layer

7. Right-click the 1st layer and choose "Create Clipping Mask"

8. Done!