jQuery.fn.rollover = function(settings){
  settings = jQuery.extend({
    suffix: "_over"  //画像ファイル名につける接尾語の初期設定値
  }, settings);
  return this.each(function(){
    var default_img = $(this).attr("src");
    if (!default_img.match((settings.suffix))) {
      var point = default_img.lastIndexOf(".");
      var mouseover_img = default_img.slice(0, point) + settings.suffix + default_img.slice(point);
      var preload_img = new Image();
      preload_img.src = mouseover_img;
      $(this).hover(
        function(){
          $(this).attr("src", mouseover_img);
        },
        function(){
          $(this).attr("src", default_img);
        }
      );
    };
  });
};