Reduce homepage image size

The homepage background image of is huge (3.3MB) and the page looks weird when trying to load it on slow networks (you only see a white background).

I tried compressing it to JPEG (lossy, I know, but the difference is almost indistinguishable from the PNG) and it comes down to around 570KB.

Thanks! I've updated the homepage header to use your reduced size image and to be a little shorter on the height.