背景画像を使わずに、ボックスの角を丸角にします。
Nifty Corners のページから、JavaScript、CSS等をダウンロードできます。

サンプル

サンプルソース

JavaScript + CSS

<link rel="stylesheet" type="text/css" href="http://www.strollnet.com/js/css/niftyCorners.css" />
<script type="text/javascript" src="http://www.strollnet.com/js/nifty.js"></script>

<script type="text/javascript">
<!--
	window.onload=function(){
	if(!NiftyCheck())
	return;
	RoundedTop("div#nifty_box","#eee","#333");
	RoundedBottom("div#nifty_box","#eee","#333");
	RoundedTop("div.nifty_gradient","#eee","#B8B8B8");
	RoundedBottom("div.nifty_gradient","#eee","#fff");
	// Rounded("div#ボックスのID","外側の背景色","ボックス内背景色");
	}
//-->
</script>

<style type="text/css">
	div#def_box {margin:10px; background: #333;}
	div#nifty_box {margin:10px; background: #333;}
	div.def_gradient{background: #fff url(http://www.strollnet.com/js/img/gradient.png) repeat-x top;margin: 10px;}
	div.nifty_gradient{background: #fff url(http://www.strollnet.com/js/img/gradient.png) repeat-x top;margin: 10px;}
</style>

HTML

<div class="posc">
	<div id="def_box">
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	</div>
	<p style="text-align:center;"><img src="http://www.strollnet.com/js/img/next.gif" /></p>
	<div id="nifty_box">
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	</div>
	<br />
	<div class="def_gradient">
		<p>
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		</p>
	</div>
	<p style="text-align:center;"><img src="http://www.strollnet.com/js/img/next.gif" /></p>
	<div class="nifty_gradient">
		<p>
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		</p>
	</div>
</div>