How does this website http://www.rallypoint.com scale their background pictures?
- As I change the size of my browser I can see the height css attribute on the div element changing so I’m assuming it’s some kind of javascript working in the background.

- The javascript function I see in the body is new Slider however I can’t find any reference anywhere on the web about this except JQuery UI Slider which does not seem relevant, is that a custom function they wrote?

They use the ‘background-size’ CSS3 attribute:
Read more about it: http://www.css3.info/preview/background-size/