Hide text button! Plese help to finish it!

Well, this is a simple code that hides the text! So please help me to change the plus.gif image to minus.gif image on click
Thanks for help


<script>
function op(obj) {
x=document.getElementById(obj);
if(x.style.display == "none") x.style.display = "block";
else x.style.display = "none"
}
</script>

<a href="Internet">Internet</a>
<a href="#" onClick="op('div1'); return false;">
<img src="plus.gif" style="height: 10px; width: 10px; border: medium none"></a><ul>
<div id="div1" style="display:none">
<li><a href="Internet Portals">Internet Portals</a></li>
<li><a href="Internet Access">Internet Access</a></li>
<li><a href="Internet Cafe">Internet Cafe</a></li>
<li><a href="Internet Shop">Internet Shop</a></li>
</div>
</ul>

You can give the <img> an id so you can access it with getElementById() to change the value of the <img>'s src attribute.

You could use a CSS sprite, so that you only need to change a class name to show the other part of it.