https://www.jb51.net/jiaoben/591575.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>立即显示上传图片</title>
<script type="text/javascript">
function showpic(){
//获取上传文件的信息
var pic =document.getElementById('pic').files[0];
//在pic的文件里抓取该文件用于显示二进制信息
var sr = window.URL.createObjectURL(pic);
//设置src属性，显示图片
document.getElementsByTagName('img')[0].src=sr;
}
</script>
</head>
<body>
<h2>立即显示上传图片</h2>
用户名：<input type="text" name="username"><br/>
头像：<input type="file" name="img" id="pic" onchange="showpic()"/>
<img src="" alt="" width="260" height="
200"/>
<br/>
<input type="button" value="注册" >
</body>
</html>