I spent some time in the script. From what I can tell, It doesn't write the images at all.
The twisties functionality appears to rely on the style sheet for that.
I believe following the model of the CSS in the image in my initial post that something like this should work:
But it does nothing.
What are the x-open-image and x-close-image statements? I understand that they're loading the triangle images used by the twisty when the content is open or closed, but those don't look like regular CSS. A search on the internet doesn't return anything either.
Are these RH-specific conventions? Or some odd subset of CSS? My guess is RH-specific. But if so, what's converting it to actual usable CSS to displays the image?
Sorry I can't answer that, beyond my pay grade. Willam is the man for such questions and as his focus has changed, he no longer follows these forums.
As you have dealt with him directly, I suggest you make contact via his site. He will be able to tell you whether or not your objective is achievable and maybe point you in the right direction.
See www.grainge.org for RoboHelp and Authoring information
In Multiscreen HTML output, the images are actually written in to the topic page, so I think the css code is just the (maybe proprietary?) reference RH uses to locate them during the generation process.