window.onload = rolloverInit;

function rolloverInit(){
for(var i=0; i<document.images.length; i++){
				if(document.images[i].parentNode.tagName == "A"){
					setupRollover(document.images[i]);
					}
			}
}
function setupRollover(thisImage){
				 thisImage.outImage = new Image();
				 thisImage.outImage.src = thisImage.src;
				 thisImage.onmouseout = rollOut;

				 thisImage.overImage = new Image();
				 thisImage.overImage.src = "images/" + thisImage.id + "_on.gif";
				 thisImage.onmouseover = rollOver;
}

function rollOver(){
this.src = this.overImage.src;
}

function rollOut(){
this.src = this.outImage.src;
}