title: Can't use css to replace background image with Bootstrap 3
body: <p>using Bootstrap 3, I'm trying to substitute a smaller image for a larger for small devices.</p>
  
  <p>With this code I get nothing. If I put the image src in the html, then I see both images in certain views. Do I have to specify a div size for this to work?</p>
  
  <p>html:</p>
  
  <pre><code>    &lt;div&gt;&lt;img id="logo"&gt;&lt;/div&gt;
  </code></pre>
  
  <p>css:</p>
  
  <pre><code>    #logo {background-image: url('../images/logo.svg'); }
  
  @media only screen and (max-width: 420px) {
      #logo {background-image: url('../images/logo-small.svg'); }
      }
  </code></pre>