Custom thumbnails for map scripts

I have a random map mod called Metamar that’s loosely based on Nomad. So for the map select thumbnail and loading screen preview I’m just using Nomad’s images for now. But I’d really like to be able to use custom ones, because Nomad’s images don’t accurately represent my map.

In the XML file I see imagepath and loadBackground, but these point to nonexistent files (presumably they’re part of some internal assets bundle). I tried changing them to point to images sitting in my mod folder, but it didn’t work. Am I missing something or is this not possible yet?