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(){
$(":image").mouseover(function(){
$(this).attr("src","img/submit_on.gif");
}).mouseout(function(){
$(this).attr("src","img/submit.gif")
});
});
</script>
<body>
<dl>
<dt>お名前<em>（必須）</em></dt>
<dd><input type="text" name="name"></dd>
<dt>コメント</dt>
<dd>
<textarea name="comment"></textarea>
</dd>
<dd><input type="image" src="img/submit.gif" alt="送信"></dd>
</body>
</html>