How to make a button look disabled

Hopefully this isn't too specific for here. I have the following button image that I use on my site

enter image description here

Here's an example with text:

enter image description here

This is rendered using the following CSS:

input.borderlessButton {
    background: url("../Images/button.png") no-repeat scroll 0 0
}

In the HTML it's getting disabled 

<input type="submit" disabled/>

This, obviously doesn't change the look as the image doesn't change. I can add a CSS :disabled pseudo class.

But how's the best way to make the button above look disabled?

All suggestions welcome.

Tags: , , , , , ,