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

HTML Code:
function op(obj) { 
if(x.style.display == "none") x.style.display = "block"; 
else x.style.display = "none" 
<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>