I think your on to the right idea with having two separate images if you want the button to appear depressed. I don't believe there is an easy way to make the buttons appear to be depressed when the user clicks it without a second image.
As far as having the button light up you could add a lighting affect to the button like glow or a spotlight that you could turn on and off when the user clicks the button. This wouldn't change the image of the button but would give an effect that was visible when the button was clicked.
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