我是习惯直接把自己的源代码拷贝，这样你可以直接自己试验，效果更明显。必要的地方我也加注释！
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Language" content="utf-8" />
<meta name="robots" content="all" />
<meta name="author" content="Tencent-ISRD" />
<meta name="Copyright" content="Tencent" />
<!--内部样式表，设置显示效果（透明度）-->
<style>
img.opacity1 { opacity:0.25;}
img.opacity2 { opacity:0.50;}
img.opacity3 { opacity:0.75;}
</style>
<title>opacity</title>
</head>
<body>
<div><img src="http://imgcache.qq.com/ac/www_tencent/zh-cn/images/sitelogo_zh-cn.gif" class="opacity1" alt="tencent" /></div>
<div><img src="http://imgcache.qq.com/ac/www_tencent/zh-cn/images/sitelogo_zh-cn.gif" class="opacity2" alt="tencent" /></div>
<div><img src="http://imgcache.qq.com/ac/www_tencent/zh-cn/images/sitelogo_zh-cn.gif" class="opacity3" alt="tencent" /></div>
</body>
</html>