Wilshere,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style>
img.big{
height: 90%;
width: 90%;
}
</style>
</head>
<body>
<img id="art11" width="250" height="250" src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" class="art11" onclick="this.classList.toggle('big')">
</body>
</html>