I had a quick look in TaharezLook and it seems we're not drawing the image for the disabled state bur rather using the normal rendering section again and applying some colours - so that's why it's not showing up for TaharezLook - if you've based your own skins on what we do in there, then presumably you have the same issue.
The solution is either:
1) Create a new ImagerySection 'disabled', which is a copy of the ImagerySection 'normal' and replace the reference to "NormalImage" with "DisabledImage" and finally change the Section specification for the Disabled state from "normal" to "disabled".
- or -
2) Split the "normal" ImagerySection into two - one for the main imagery and the other for the custom Image (say "normal" and "normal_image"), add a new ImagerySection based off of the new "normal_image" that instead references the DisabaledImage and name the ImagerySection "disabled_image", add Section specifications to the Normal and Disabled states that reference the appropriate ImagerySection for the desired image, so "normal_image" and "disabled_image" respectively.
I'll fix our versions of these either tonight or some time tomorrow, thanks for bringing it up

HTH
CE