$(function()  
{  
        var targetImgs = $('img');  
  
        targetImgs.each(function()  
        {  
            if(this.src.match('_f'))  
            {  
  
                this.rollOverImg = new Image();  
                this.rollOverImg.src = this.getAttribute("src").replace("_f", "_o");  
                $(this.rollOverImg).css({position: 'absolute', opacity: 0});  
                $(this).before(this.rollOverImg);  
  
                //ここいらないのなら削除してください。  
                $(this.rollOverImg).mousedown(function(){  
                    $(this).stop().animate({opacity: 0}, {duration: 0, queue: false});  
                });  
  
                $(this.rollOverImg).hover(function(){  
                    $(this).animate({opacity: 1}, {duration: 400, queue: false});  
                },  
                function(){  
                    $(this).animate({opacity: 0}, {duration: 400, queue: false});  
                });  
  
            }  
        });  
});