title: Darkening a CSS background image
body: <p>I'd like to darken a background picture. I had found that I have to use opacity so I wrote my css code:</p>
  
  <pre><code>body {
          width: 960px;
          margin: 0 auto;
          font-family: Georgia, Times, serif;
          background:
          linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0.5)),
          url('../pics/parachute.jpg');}
  </code></pre>
  
  <p>Everything works fine! However when I added some flags to make a full screen background picture, my effect disappeared.</p>
  
  <pre><code>body {
          width: 960px;
          margin: 0 auto;
          font-family: Georgia, Times, serif;
          background:
          linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0.5)),
          url('../pics/parachute.jpg');
          -webkit-background-size: cover;
          -moz-background-size:  cover;
          -o-background-size: cover;
           background-size: cover;}
  </code></pre>
  
  <p>What is the reason that it doesn't work?</p>