;((function(){
    Element.implement({
        parallax: function(){
            var height, self = this,
                firstTop = this.getPosition().y,
                pos, top;

            var update = function(){
                pos = window.getScroll().y;
                top = self.getPosition().y;
                height = self.getSize().y;
                speedFactor = self.get('data-parallax-delta') || 0.7;

                if (top + height < pos || top > pos + window.getSize().y) {
                    return;
                }

                self.style.backgroundPosition = "0 " + -(Math.round((firstTop - pos) * speedFactor)) + "px";
            }

            RokScrollEvents.push(update);
            update();
        }
    });

    window.addEvent('load', function(){
        document.getElements('.rt-parallax').each(function(element){
            element.parallax();
        });
    });
})());

speedFactor = self.get('data-parallax-delta') || 0.7;

speedFactor = -0.5;

document.getElements('.rt-parallax').each(function(element){

document.getElements('.[YOUR_CSS_BODY_CLASS]').each(function(element){

var RokScrollEvents = [], RTScroll = function(){
            if (!RokScrollEvents.length) window.removeEvent('scroll', RTScroll);
            else {
                for (var i = RokScrollEvents.length - 1; i >= 0; i--){
                    RokScrollEvents[i]();
                };
            }
        };
        window.addEvent('load', function(){
        	window.addEvent('scroll', RTScroll);
        	RTScroll();
        });

.YOUR_CSS_BODY_CLASS {
    background-image: url(../../../../images/anyimage.png);
    /*overflow: hidden;*/
    background-repeat: no-repeat;
    background-position: center top;
    background-size: cover;
    background-attachment: fixed;
    min-height: 1000px;
}