[Sweet]

Например, так:

var url1 = '...', url2 = '...', 
div1 = document.getElementById('div1'), div2 = document.getElementById('div2')
setInterval( function(){
  div1.style.background = "url('"+url2+"')"
  setTimeout( function(){
    div1.style.background = "url('"+url1+"')"
  }, 1000)
}, 11000)