How to create an image that is just shading and transparency? [duplicate]

Question 1

In photoshop I created this image that is supposed to represent the folds of a curtain:

I, however, only want the shading aspect of this so that I can export that to a PNG and display it over any color on the Web.

So my question then is how do I get only the shading part of this image into a PNG?

Question 2

Open your Layers and Channels palettes;
Make your image a regular layer as opposed to a background layer. Easiest way is to double click the layer bar in the Layers palette;
In the Channels palette, make sure the RGB channels are visible, then click the leftmost icon on the bottom, ‘Load channel as selection’;
In the Layers palette, click the ‘Add Layer Mask’ icon (third from left, rectangle with a round hole in it);
With the Layer Mask still selected, choose Image > Adjustments > Invert, shortcut Control / Command + I;
Use Layer Styles to give the layer a black Color Overlay;
Create a coloured layer under your current layer to see the effect;
Tweak the effect using opacity, curves or other adjustments on both the actual layer and the mask;
Save as png-24. Don’t forget to turn off your coloured background layer before you do.

Answer