Get it here:
http://www.4shared.com/file/82582953/34 ... aster.html
With this, you could divide any sky picture into these 4 sections to create a new sky. How we generate the one we see in the Map Maker I haven't figured out yet, but its probably just a re-sized image of three or four together.
A quick test of all 4 together to give an idea of what you can do (I used a freeware program called "Sky Panorama" that doesn't seem to be available anywhere anymore, but I think any decent picture will do):
