Image from Config class data doesn't appear in site

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Hi!
I want to show the path to an image choosen in the dashboard. To display the path to it, I have done this:

<?php echo $r = $im->getThumbnail('image', 600, 80);?>

I have also tryed this:
<?php echo $r = $im->getThumbnail('image', '600', '80');?>


But it shows nothing.
Before I have loaded the image helper (in the same header.php file).

What is wrong?
Thank You.

PS: Here is the documentationhttp://www.concrete5.org/documentation/developers/files/helpers/...

marticps