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.
<body background="images/try.gif">
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.
Good luck!!!