Hmm, are you certain you can't set it to custom? It doesn't seem like that option is greyed out on your screenshot.
Edit: Ah, you want to replace the minimap, not remove it?
I think you need to import your image to the map and use either the data editor or put your image info in a certain location path within the import folder.
I can't help you right now since I'm not in front of my computer. Will try to find an solution later, if no one else will pick this up