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(){
document.querySelector("input").addEventListener("dblclick",function(){
document.querySelector("img").setAttribute("src","img/after.png");
});
});
</script>
<body>
<p><input type="button" value="変更" /></p>
<p><img src="img/before.png" alt="変更前" /></p>
</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(){
$("input").dblclick(function(){
$("img").attr("src","img/after.png").attr("alt","変更後");
});
//下の記述でも可
/*
$("input").bind("dblclick",function(){
$("img").attr("src","img/after.png").attr("alt","変更後");
});
*/
});
</script>
<body>
<p><input type="button" value="変更" /></p>
<p><img src="img/before.png" alt="変更前" /></p>
</body>
</html>