I am also trying to use tiddlymap with a world building map!
I just imported the image directly, and I am a coding noob so I cant help you. However, if you've made any progress with your project and are comfortable sharing I would love to see because I am trying to do the same thing.
Maybe I could even make a "use tiddlywiki for worldbuilding or DnD" tutorial for people once I've figured this out. Right not I am stuck trying to figure out how to create a max bounds to my background image.
Have a nice day :)
H