如果想使用图像作为提交按钮，你就需要将<input>元素的type特性的值设置为image.sec,width,height和alt特性的用法和它们在<img>元素中的用法是一样的。
<!DOCTYPE html>
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<html>
<head>
<title>TODO supply a title</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div>TODO write content</div>
<form action="http://www.example.org/subscribe.php">
<p>发送到服务器的表单:</p>
<input type="text" name="email" />
<input type="image" src="images/subscribe.jpg" width="100" height="20" />
</form>
</body>
</html>