Did you see this post - Dynamic Images: Displaying in a dashboard an image (photo or picture) linked to a filter or sheet - it might help in this case. If not, then the idea you have linked already is the best place to vote for such a feature.
I hope that helps
I have no definite answer but I really support your issue. I am trying to do the same. I did successfully manage to achieve the result you want by modifying the XML with a python script. Basically there are tags of this kind that you have to fill:
<map to="image path"> <bucket> label </bucket> </map>
It does the job but it's not fantastic because it cannot be fully automatised. In fact the script only does the shapes "assignation" to the labels while the loading into Tableau still needs to be done by manually clicking "Reload shapes" on Tableau. Also I warn you that we aucceded with 660 images but we tried with 1800 and the Desktop crashed. So this might be a limit too. What I am tring now is to use extensions and load the images via URLs but I'm not sure the shapes are currently acessible in the extensions API.
I'd be curious to speak with someone that knows the extensions API well.