For the life of me I can't figure out how to change art for Collections

[theguymadmax]

To need to unlock the collection first if you want to edit the images manually. Just don't try to identify the movie, there's a bug that deletes the file entries in the xml file. All collections are saved in xml files in your config/data/collections folder. I recommend you backup that folder.

Demonstrated here:
https://github.com/jellyfin/jellyfin/issues/12724