function preload(imgName)
{
	var a = new Image();
	a.src = imgName;
	return a;
}
		
function rollOn(imgName) 
{
	if (document.images) 
	{
		eval('document.' + imgName).src = eval(imgName + "_on.src");
	}
}
 
function rollOff(imgName) 
{
	if (document.images) 
	{
		eval('document.' + imgName).src = eval(imgName + "_off.src");
	}
}