I'm not familiar with the details of AD but looking at the AD example for local.protected.php, I'm wondering whether your problem is with the settings which control the url. This doesn't strike me as a permissions issue.
You should be able to access your image through the media setting of fetch. For instance, if you have this:
my_wiki/lib/exe/fetch.php?w=700&h=495&cache=cache&media=my_image_info
then create a new url like this, using only the media parameter:
http://your_domain/my_wiki/lib/exe/fetch.php?media=my_image_info
Plug this into your browser and see if you get your image. Probably not. Then see what the fetch parameters are when you don't use AD, where you say it works. Then plug that into the browser; it should get your image.
A comparison of the two might help you to get the correct AD settings in local.protected.php, if that's the problem.