[Change background on scroll]

So this is awesome - but if I want to fade change the background image, rather than colour of a section, what do I need to do?

 

I'm using this to change the background image on section 2, and it changes the image, but it's quite sudden:

 

gsap.to("body", {
backgroundImage:'url(/img/image.jpg)',
  immediateRender: false,
  scrollTrigger: {
    trigger: ".section-2",
    scroller: ".container",
    scrub: true,
    start:'top bottom',
    end: '+=100%'
  }
});