$(document).ready( function() {
   $(".thumbnail1 img").mouseover( function() {
       var changeSrc = this.src.replace("_s.jpg","_b.jpg");
       $("#target1").fadeOut(
           "slow",
           function() {
               $(this).attr("src", changeSrc);
               $(this).fadeIn();
           }
       );
   });
   
   $(".thumbnail2 img").mouseover( function() {
       var changeSrc = this.src.replace("_s.jpg","_b.jpg");
       $("#target2").fadeOut(
           "slow",
           function() {
               $(this).attr("src", changeSrc);
               $(this).fadeIn();
           }
       );
   });
   
});