setEvents = function() {
    nRoot = document.getElementById("topmenu");
    for (i=0; i<nRoot.childNodes.length; i++) 
    {
        node = nRoot.childNodes[i];
        if (node.nodeName=="A") 
        {
            //alert(node.childNodes[0].childNodes[0].nodeName);
       
            if(node.childNodes[0].src.indexOf('_a.gif') != -1)
            {
	            node.onmouseover=function() {
	                this.childNodes[0].src = this.childNodes[0].src.replace('_a.gif', '_b.gif');
	            }
	         
	            node.onmouseout=function() {
	                this.childNodes[0].src = this.childNodes[0].src.replace('_b.gif', '_a.gif');
	            }
           	}
        }
    }
}
window.onload=setEvents;