[cyber]

вот так

function FindReplace (word, img) {       

  var elements = document.body.getElementsByTagName('*');
  var element, html, i = 0, tagN;
  var regE = new RegExp (word,'ig');
  var leng = elements.length;
  (function closing() {
   
    element = elements[i];
    tagN = element.tagName.toLowerCase();
    
     if ( tagN == 'script' || tagN == 'style')end();
    
    html = element.innerHTML; 
   
    if( html.search(regE)== -1) end();
    
     
    element.innerHTML =  html.replace(regE,img);  
      
       
    function end () {
     if (i > leng-1) return; 
      console.log(i > leng-1)
     i++;
     setTimeout(closing,0);
     }
   end(); 
  }());

    }
FindReplace('lol','<img src="http://ctrlc.ru/wp-content/uploads/2012/06/18.jpg" >')