Level 11 haven’t got much into it, just a look in right place will do it. After checking out the source code you will find things similar to the previous levels.
But it does not work out and us gives only an image with Nice try written on it. This isn’t much of a clue, how about a second look at the source code? In the second look I looked more thoroughly and found this meta tag with the answer.
<meta name="robots" content="goto: clipart.php">
Another directory traversal to reach level 12.