DuneX indeed produce an XML file, but it uses to store its library AND movie info. This XML resides in the DuneX configuration folder, usually in your documents folder.
Images produced by DuneX (setting export for TVixie) are called folder.jpg (the cover) and tvix.jpg (the background).
You need to rename tvix.jpg into about.jpg in order to let the mede8er recognize it.
The real problem is that you'll miss the aMovieName.xml file that mede8er uses to display info in jukebox mode and to produce the mede8er.db when you scan the folder/favourite.
So, what you need (maybe me too) is an application that reads DuneX xml library, adjust images name according to mede8er standard and produces a movie info XML for each movie, named and placed accordingly.
If there enough request of such a program I may try and write down some code for OSX...