$(function(){
	$("img.rollover").mouseover(function(){
		$(this).attr("src",$(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(){
		$("").attr("src",$(this).attr("src").replace(/^(.+)(\.[a-z]+)$/,"$1_on$2"));
	});
});

プラグイン化

下のソースに書き換えて「jquery.rollover.js」に保存

$.fn.rollover = function(){
	$(this).mouseover(function(){
		$(this).attr("src",$(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(){
		$("").attr("src",$(this).attr("src").replace(/^(.+)(\.[a-z]+)$/,"$1_on$2"));
	});
	return this;
}