Can not save with aviary

Question

I am facing a problem because I can't save my edited image, I put this code in index.php

<?php
    $image_data = file_get_contents($_REQUEST["Url"]);
    file_put_contents("photo.jpg",$image_data);
?>

But I have an error :

 Warning: file_get_contents() [function.file-get-contents]: Filename cannot be empty in index.php on line 40

If I put the direct link to the edited image it works.

Regards