Some small performance improvements

This PR introduces three minimal performance optimizations to the site (happy to submit a few more).

1. Shave 1.2MB off the images shipped down the line.
2. Swap out the ~5MB gif on the homepage for a static header. This improves both the Speed Index of the site as well as how quickly it is visually complete. Also saves us roaming data users some money :) If you really really wanted to keep it, we could prolly just turn it off for users with narrow viewports with a media query.
3. Ensure minification is switched on for Sass/Compass

Other optimizations I can PR back:

* Optimize for the critical rendering path
* Remove unused CSS
* Add in additional minification (e.g HTML)
* Add in performance measurement as part of CI

These may require switching over to some more modern tooling options for your build, just for
ease of integration, but could prolly also just use Make.