W3C的download属性，亲测只适用于PC和移动端浏览器，打包成app无效
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
.box{border: 1px solid black;padding:0 10px;}
img{width:200px;}
</style>
</head>
<body>
<!--W3C的点击按钮下载图片-->
<span class="box"><a href="adfagaga.jpg" download="w3logo">保存</a></span><br />
<img src="adfagaga.jpg" alt="W3School"/>
</body>
</html>