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