An ebook is just bits, so it *shouldn't* matter if you put more than one image in. My suggestion would be to put the whole image in, then ALSO put in split, magnified images. And make sure what you post on your website is the highest definition you can obtain - anyone can scale it down, but if you only have small images, then you can't enlarge it to read a small name, for example. If you put the split images next to each other, then as you page from one to the next you should see the adjoining map. Kinda like turning the page on a Thomas guide map of your local city.
I was just reading David Weber's latest Safehold novel (although for some reason I keep ending up with his Tor books in paper instead of ebook....) and I was constantly referring to the maps, trying to figure out where everything was. If I had it in bits, I'd want to bookmark it, and jump back and forth to see the details.