Couldn't you divide it into sections and only load the parts on-screen or nearby? That would keep the size down. I dunno, though, maybe that won't work for whatever you're planning. Worth suggesting, at least.
Well, here's the thing: the 3200x3200 graphic was only for North America.
Oh, planet Earth. Why are you so large?
I'm sure I can cook-up some sort of plan B.