Fade In Css Background-Image only

Fade In Css Background-Image only

I'm making a landing page and wanted to fade a css background image (actually  2 of that part of my divs)

<body>

<div id="outer_wrapper">
   <div id="inner_wrapper">

   <div class="content_info"></div>

  </div>
</div>

separate css file contains the following
div#outer_wrapper {background:transparent url(sitename.com/image.jpg) bottom left no-repeat; }
div#inner_wrapper {background:transparent url(sitename.com/image.jpg) bottom left no-repeat; }

I couldn't find an exact example or of a way to do this. I've been struggling on this for hours. 

All I'm trying to do is fade in those divs after a certain time has passed (like 5secs after the page loads)

Can anyone help out please.

Thanks!