[nerbert]

If you look at the sprite image you'll see there are three versions of each image. The most faded corresponds to the disabled state for the button; I'm not sure about the intermediate one. So first you would have to use photoshop or equivalent to make faded versions of your image and then carefully copy (or "clone") in your own images over the top of the existing images. If you get it correctly positioned, down to the pixel, it will work. The only alternative I know is to generate a whole new sprite image but to do that you would need every one of the other images, and then you would have to replace the css somewhere to match up with the new image.

What I suggested here is doable if you have a good photo/image app and are willing to do some trial and error with positioning.