I need to center a image inside a div both vertically and horizontally. I am using the display inline-block property.
HTML
        
                      
      You can put the image into a table like this:
  <table class="container">
     <tr class="box">
         <td class="image-container"><img src="image.gif"></td>
     </tr>
  </table>
And then use this in your CSS:
.image-container {
   vertical-align: middle;
   text-align: center;
}
JSFiddle
This approach works perfectly for dynamic content of unknown dimensions.
jsFiddle example
Basically, we are using vertical-align:middle for vertical centering, and text-align:center for horizontal centering. In order for vertical-align:middle to work on a div, we need to set the parent elements to display:table, and the targeted child div to display:table-cell. Sure you could achieve such centering with absolute positoning, however, that would require you to know the dimensions of the img. This approach works for unknown dimensions.
HTML
<div class="container">
   <div class="box">
       <img src="http://placehold.it/200x200">
   </div>
</div>
CSS
html, body {
    height:100%;
    width:100%;
    margin:0px;
}
.container {
    display:table;
    vertical-align:middle;
    text-align:center;
    height:100%;
    width:100%;
}
.box {
    display:table-cell;
    vertical-align:middle;
}
for the horizantal you can use:
.box {
display: inline-block;
width: 32px;
height: 32px;
border: 1px solid #f2f2f2;  
margin:auto;
text-align:center;
}
for the vertical as far as i know you can use
#content {position:absolute; top:50%; height:240px; margin-top:-120px; /* negative half of the height */}
please read: http://blog.themeforest.net/tutorials/vertical-centering-with-css/