解答例

<!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(){
	$("img").click(function(){
		$("img").addClass("enlarge");
	});
});
</script>
<style>
img{
	display:block;
	width:200px;
	height:200px;
	cursor:pointer;

	/*これらを記述するとアニメーションが加わります*/
	/*
	-moz-transition-property:width, height;
	-webkit-transition-property:width, height;
	-o-transition-property:width, height;
	-ms-transition-property:width, height;

	-moz-transition-duration:500ms;
	-webkit-transition-duration:500ms;
	-o-transition-duration:500ms;
	-ms-transition-duration:500ms;

	-moz-transition-timing-function:ease-in-out;
	-webkit-transition-timing-function:ease-in-out;
	-o-transition-timing-function:ease-in-out;
	-ms-transition-timing-function:ease-in-out;
	*/
}
.enlarge{
	display:block;
	width:400px;
	height:400px;
	cursor:pointer;
}
</style>
</head>
<body>
	<img src="img/enlarge.png">
</body>
</html>

inserted by FC2 system