how to call action with guid correctly?

i've been creating a script to resize all existing tidypics thumbnails to a new size after the size is changed in the admin section; presently there is only the option to change them one at a time.

i found the action: photos/admin/create_thumbnails

and it is used via ajax.. 

i am wanting to call it from php directly using an image guid.

i used so far:

$url = elgg_add_action_tokens_to_url('photos/admin/create_thumbnails?guid=' . $entity->getGUID());

this redirects the browser to the homepage and does not process the thumbnail..

anyone know what i'm missing?