function vis()
{
$("#webcam").image("http://www.caorle.com/webcam-piazza-vescovado/current.jpg?"+Math.random(),function(){});
}

$(function()
{

$.fn.image = function(src, f){ 
   return this.each(function(){ 
     var i = new Image(); 
     i.src = src; 
     i.onload = f; 
     this.src= src;
   }); 
 } 

var test=setInterval("vis()",3000);

});