var my=900;
var dy=-1;
var h=125+40/2;
var y=my-h;

function animate() {
 y+=dy;
 if(y+h>my) {y=my-h; dy=-dy;}
 else if(y<0) {y=0; dy=-dy}
 document.getElementById('header').style.backgroundPosition='0 '+(y+h-my)+'px';
 setTimeout('animate()',25);
}

window.onload=animate;