Fade in high resolution image, where body is initially set to low resolution image.

background-loader.js

jQuery(function ($) {
    var newpic = new Image();
    newpic.onload = function()
    {
        var $div = $('<div id="temp-background"/>').css({
            'background-image': 'url(low-resolution-image.jpg)',
            'position': 'absolute',
            'height' :'100%',
            'width' :'100%',
            'z-index' :'-1',
            'top' :'0px',
            'left' :'0px',
        });
        $('body').append($div);
        $("body").css('background-image', 'url(high-resolution-image.jpg)');
        $('#temp-background').fadeOut(1000);           
    }; 
    newpic.src = 'high-resolution-image.jpg';
});