[Ballbarian]

Try this llamabeast:
Create a cross-platform program that your users can download (or run from a browser) that will change every pixel to black except for the pure white province markers. Then have them upload the resulting image.

I have attached a "minimized" version of the silent seas map image. The file size goes from 2787 KB to 50 KB. If you are able to automate zip file decompression as well, have the user or your distributed application zip the resulting "minimized" image and it will squeeze down to 2 KB.

I have not tested the map image in game yet, but I see no reason why it should not work. I have attached it to this post for anyone who wants to try it out.

Hope this helps!