I made an account just to post this... lol. Anyway, I was able to get a custom dock image working on my 10.8 Mountain Lion GM build.
So basically what you do, is download dockdesigner here:
http://www.moritzwette.com/dockdesigner/dockdesigner.zip?forcedownload
Open up dockdesigner and click the 3D tab, then click the "NO" button under transparent dock. After that you can copy and paste in your custom dock images; the ones that I highlighted.
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So basically what you do, is download dockdesigner here:
http://www.moritzwette.com/dockdesigner/dockdesigner.zip?forcedownload
Open up dockdesigner and click the 3D tab, then click the "NO" button under transparent dock. After that you can copy and paste in your custom dock images; the ones that I highlighted.