$('.rollover').mouseover(function(){
	if(!$(this).attr("src").match(/^(.+)_on(\.[a-z]+)$/, "$1_on$2")){
		$(this).attr("src",$(this).attr("src").replace(/^(.+)(\.[a-z]+)$/, "$1_on$2"));
	}
}).mouseout(function(){
	$(this).attr("src",$(this).attr("src").replace(/^(.+)_on(\.[a-z]+)$/, "$1$2"));
})
.each(function(){
	$("<img>").attr("src",$(this).attr("src").replace(/^(.+)(\.[a-z]+)$/, "$1_on$2"));
})