Apparently that feature was only added in tgui 0.8-dev, according to the commit history it was one of the first big features added after I released 0.7.
If upgrading is not an option then a workaround could be to call button->getRenderer()->setNormalTexture(...) each time you disable or enable it (to swap which image is displayed).
Also, even though 0.8-dev has BackgroundColorDisabled and TextureDisabled properties, they are not being used yet by the default themes. So you will have to edit the theme file yourself (and provide your own image if you use TextureDisabled).
There is currently no list of properties outside the source code. Most properties are used in one of the 3 theme files shipped with TGUI, for the others you will have to keep checking the source code (either from the src folder or from the tests).
I intended to put a list on the website some time ago already, but I haven't had the time yet. It currently has low priority.
