var step = 1; 				// How many pixels to move per step
var current = 0;			// The current pixel row
var imageHeight = 4300;		// Background image height
var headerHeight = 300;		// How tall the header is.
var scrollSpeed = 40;

var restartPosition = -(imageHeight - headerHeight);

function scrollBg() {
  current -= step;
	
  if (current == restartPosition){
	 current = 0;
  }

  $('#header').css("background-position","0 "+current+"px");
}

//Calls the scrolling function repeatedly
var init = setInterval("scrollBg()", scrollSpeed);