Instead of using onmouseover, onmouseout in each image, use the following function.

Remember to name the images in specific format.

$("img.rollover").hover(
	function() { $(this).attr("src", $(this).attr("src").split("_s.").join("_o.")); },
	function() { $(this).attr("src", $(this).attr("src").split("_o.").join("_s.")); }
);