Convert the single-tile Earth image into tiled imagery.

Per Kevin's recommendation earlier, swap out the single-tile image for tiles of the same resolution.  Overall file size is a couple percent smaller, and visual quality is the same or a tiny bit improved.  Colors darkened for contrast.

The license is the same (Natural Earth II, Public Domain, modifications allowed, no credit needed).  It is already included with LICENSE.md so there is no change there.