function setBold(num) {
	var element = $("line"+num);
	toggleValues(element.style.fontWeight,"bold","normal") 
	alert(element.style.fontWeight);
	return false;
}

window.onload = function() {
	$("line1-bold").onclick = setBold ;
}	

<input type="image" src="/images/buttons/text-bold.jpg" id="line1-bold" alt="Bold" width="17" height="17" />