$(document).ready(function() {
$(".rollover").mouseover(mouse_over);
$(".rollover").mouseout(mouse_out);
});

function mouse_over() {
var current_img;
var rollover_img;
current_img = this.firstChild.src;
rollover_img = current_img.replace(".jpg","_over.jpg");
this.firstChild.src =  rollover_img;
}

function mouse_out() {
var current_img;
var rolloff_img;
current_img = this.firstChild.src;
rolloff_img = current_img.replace("_over.jpg",".jpg");
this.firstChild.src =  rolloff_img;
}