var now = new Date();

   var sec = now.getSeconds();

   var ses = 30;     
   
   rnd_number = Math.floor(sec/ses);

if (rnd_number == 0)
   document.write('<img border="0" src="sfondo3.gif">');
if (rnd_number == 1)
   document.write('<img border="0" src="sfondo3.gif">');