$(document).ready(function(){
$("img.rollover").each(function() {
var off = this.src;
var dot = this.src.lastIndexOf('.');
var on = this.src.substr(0, dot) +
'_on' + this.src.substr(dot, 4);
$(this).hover(
function() { this.src = on; },
function() { this.src = off; });
});
});