HTML5
<!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>
window.addEventListener("load",function(){
//書き方①
document.querySelector("#first a").addEventListener("click",function(){
document.querySelector("#first img").setAttribute("src","img/after.png");
});
//書き方②
document.querySelector("#second a").addEventListener("click",function(e){
document.querySelector("#second img").setAttribute("src",document.querySelector("#second a").getAttribute("href"));
e.preventDefault();
});
});
</script>
<body>
<div id="first">
<p><a href="javascript:void(0)">画像を変更</a></p>
<p><img src="img/before.png" alt="変更前" /></p>
</div>
<div id="second">
<p><a href="img/after.png">画像を変更</a></p>
<p><img src="img/before.png" alt="変更前" /></p>
</div>
</body>
</html>
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(){
//書き方①
$("#first a").click(function(){
$("#first img").attr("src","img/after.png").attr("alt","変更後");
});
//下の記述でも可
/*
$("#first a").bind("click",function(){
$("#first img").attr("src","img/after.png").attr("alt","変更後");
});
*/
//書き方②
$("#second a").click(function(){
$("#second img").attr("src",$(this).attr("href")).attr("alt","変更後");
return false;
});
//下の記述でも可
/*
$("#second a").bind("click",function(){
$("#second img").attr("src",$(this).attr("href")).attr("alt","変更後");
return false;
});
*/
});
</script>
<body>
<div id="first">
<p><a href="javascript:void(0)">画像を変更</a></p>
<p><img src="img/before.png" alt="変更前" /></p>
</div>
<div id="second">
<p><a href="img/after.png">画像を変更</a></p>
<p><img src="img/before.png" alt="変更前" /></p>
</div>
</body>
</html>