Run all PNG assets through ImageOptim

Here is a little improvement on page weight / load time: all PNG images have been losslessly compressed with various compression tools (via [ImageOptim](http://imageoptim.com/)).

Impact on page load has not been benchmarked, but here is a screenshot of the gain on image size.

![nodeschool-imageoptim](https://f.cloud.github.com/assets/877585/2492375/ce764d46-b226-11e3-9379-73c37528d3f3.PNG)