Center AdSense Above Header and Add to Child Theme

I updated my WordPress site to 3.4.2 and have now created a child theme since I lost several modifications. Here’s the site: http://reddoorreview.com

I’m trying to:
1. Ad my AdSense ad at the very top of my site back,
2. Center it horizontally,
3. And make sure that it doesn’t disappear when I update WordPress again.

I’ve gotten it to appear again, but not through the child theme. How do I do that (since the child theme is CSS and I’m looking to add details to the header.php file)?

Plus, I got it to center before, but it isn’t centering horizontally any more. Suggestions?

On header.php:

<img class="displayed">
<script type="text/javascript"><!--
google_ad_client = "ca-pub-5427562956145395";
/* Skyad */
google_ad_slot = "0192440898";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</img>

On the child theme:

IMG.displayed {
    display: block;
    margin-left: auto;
    margin-right: auto
}