I'm working on updating the google map to include the new beach town and whatnot. I decided to abandon Overviewer due to a lack of updates, however, and now I'm trying to get the settings right in the new renderer (Tectonicus). Since I have to re-render the whole thing every time I try something new, this takes a ridiculously long time. But the end result will be worth it!
Other settings aside, these renderers are very limited in their compression options. JPG seems to work best, but I can't set the quality within the renderer. My solution to this is to use a batch scrip to render the map as lossless PNG, run ImageMagick to convert the end result to JPG and lastly change the file extensions back to PNG so the file paths match the originals. Sure, it's not an elegant solution, but it should give us the a much better size/quality ratio, and lord knows we need it for this behemoth of a map.