Optimize images

map.png from 317kb to 75kb
intro-bg.jpg from 419kb to 232kb

map.png was exported in PNG-8 (huge file size improvement) and
intro-bg.jpg was reduced a little bit in quality (almost unnoticeable)
and saved it as a progressive jpg, which does *not* load sequentially
from top to bottom, giving a faster feeling to the end user.