JSFiddle - React, Tailwind, and code Playground

CSS

body {  
    background-image: url('http://placehold.it/350x250&text=background-1');
    background-repeat:no-repeat;
    background-attachment:fixed;
    background-size:cover;
    height:3000px;
}

JavaScript

var a = 'http://placehold.it/350x250&text=background-1',
    b = 'http://placehold.it/350x250&text=background-2';

var newImg = new Image();
newImg.src = b;

$(window).scroll(function () {
    if ($(this).scrollTop() > 800) {
        $('body').css({backgroundImage: 'url("'+b+'")'});
    }else{
        $('body').css({backgroundImage:'url("'+a+'")'});
    }
});