jQuery.fn.fadeBW = function() {
         // faire quelque chose
         
	 var string_stop=this.children("img").attr("src").length - 7;
	 var img_bw = "url(" + this.children("img").attr("src").substring( 0,string_stop) + ".jpg)";
	 this.css("background-image", img_bw);
	 
         // Your code goes here 
            this.hover(function(){
                $(this).children("img").animate({opacity: 0}, 800 );},
                function(){
                $(this).children("img").animate({opacity: 1}, 800 )
            });     
 };