A few updates to the About page

- Updated text on about-overview.png
- Changed about-typography, about-contrast, and about-modern-aesthetics from PNG24 to PNG8, saving 83 KB
- Reluctantly changed about-twentyfourteen from PNG to JPEG, because it saves a whopping 615 KB
- Tweak the headline size and styling of the about-twentyfourteen image to account for its lack of a transparent background.

See #26387.


Built from https://develop.svn.wordpress.org/trunk@26806


git-svn-id: http://core.svn.wordpress.org/trunk@26693 1a063a9b-81f0-0310-95a4-ce76da25c4cd