jQuery
<!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(){
$("#first").delegate("a", "click", function(){
$("img").attr("src",$(this).attr("href")).attr("alt",$(this).attr("title"));
return false;
});
});
</script>
</head>
<body>
<span id="first"><a href="img/after.png" title="変更後">変更後</a></span>
<span id="second"><a href="img/after.png" title="変更後">変更後</a></span>
<p><img src="img/before.png" alt="変更前"></p>
</body>
</html>