var scrollSpeed = 70; 		// Speed in milliseconds
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.

//The pixel row where to start a new loop
var restartPosition = -(imageHeight - headerHeight);

function scrollBg() {

    //Go to next pixel row.
    current -= step;

    //If at the end of the image, then go to the top.
    if (current == restartPosition) {
        current = 0;
    }

    //Set the CSS of the header.
    $('#header').css("background-position", "0 " + current + "px");


}

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