lastScrollY=0; 
function rollimg(){ 
	var diffY = document.body.scrollTop;

	if (diffY <= 0)
	{
		diffY = document.documentElement.scrollTop;
	}
	percent=.1*(diffY-lastScrollY);
	if(percent >0 )
		percent=Math.ceil(percent); 
	else
		percent=Math.floor(percent);

	var top = parseInt(document.getElementById("weblink").style.top);
	document.getElementById("weblink").style.top = (top += percent) + "px";
	lastScrollY += percent; 
}

window.setInterval("rollimg()",1);