Header image is large, uncompressed, doesn't adjust for mobile

https://developers.google.com/speed/pagespeed/insights suggests the header image should be resized and compressed.

By cutting the size by 60% in mspaint (to 1920 wide) and using an online image compressor, got it down to 150KB from almost 700KB.

I think the best thing to do would be to make the header a responsive image. https://css-tricks.com/on-responsive-images/

This can be done in PHP, or JS, or (hacky) CSS.