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(){
var img = new Image();
img.src = "img/after.png";
document.querySelector("img").addEventListener("mouseover",function(){
document.querySelector("img").setAttribute("src",img.src);
document.querySelector("img").setAttribute("alt","変更後");
});
document.querySelector("img").addEventListener("mouseout",function(){
document.querySelector("img").setAttribute("src","img/before.png");
document.querySelector("img").setAttribute("alt","変更前");
});
});
</script>
<style>
img{
cursor:pointer;
}
</style>
</head>
<body>
<img src="img/before.png" alt="変更前">
</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(){
var img = $("<img>").attr("src","img/after.png");
//下の記述は不可
//var img = $("img").attr("src","img/after.png");
$("img").mouseover(function(){
$(this).attr("src",img.attr("src")).attr("alt","変更後");
}).mouseout(function(){
$(this).attr("src","img/before.png").attr("alt","変更前");
});
});
</script>
<style>
img{
cursor:pointer;
}
</style>
</head>
<body>
<img src="img/before.png" alt="変更前">
</body>
</html>