Does it have to be a single image file? Might it be redesigned to consist of a relatively low resolution top-level file (I mean one that is sparse in the types of information that it shows. I don't mean using a lower-percentage jpg resolution), while containing links to more detailed regional images?

Alternatively, does it have to be a bitmapped image (jpg or png format)? Vector images (.svg format) can be much more efficient, depending on the image design.