<!DOCTYPE html>
<html lang="ja-JP">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,user-scalable=yes,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.5">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
$(function(){
$("input").click(function(){
$("a").html("<img>");
$("img").attr("src",$("a").attr("href"));
//下の記述でも可
//$("a").html("<img src=\"" + $("a").attr("href") + "\">");
});
});
</script>
</head>
<body>
<a href="img/show.png">リンク</a>
<input type="button" value="表示">
</body>
</html>