                        课程
                    
                        /前端开发
                        
                            /HTML/CSS
                        
                        /网页简单布局之结构与表现原则

有个问题不明白、为何不设置padding值，背景图案就显示不出来、

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>编程挑战3-3</title>
<style type="text/css">
body{ margin:0;}
div{background:#460E29;width:700px;padding:20px}
img{background:url(http://img1.sycdn.imooc.com//5385ac820001905201440133.jpg) no-repeat ;
margin:0 10px 0 0; padding: 0;}
</style>
</head>
<body>
<div>
    <img src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg"/>
    <img src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg" />
    <img src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg" />
    <img src="http://img1.sycdn.imooc.com//5385acb000013b0d00950095.jpg" /> 

</div>
</body>
</html>

只要给padding设几个值，背景图案就慢慢出来了，为何设margin值不行，