You have to also add the "navbar-brand" class to your image a container, also you have to include it inside the .navbar-inner container, like so:

<div class="navbar navbar-fixed-top">
   <div class="navbar-inner">
     <div class="container">
        <a class="navbar-brand" href="index.html"> <img src="images/57x57x300.jpg"></a>
     </div>
   </div>
 </div>