Thread:Project:Support desk/img auth.php and additional images repo/reply

Since staffimages is not part of the local file repo, you can't use thumb.php to get URLs from it.

Apparently, you'll need to create a new FileBackend class for your extension and define the storage structure. Looking at the img_auth.php file that's the only way it could work. See the comment Various extensions may have their own backends that need access. You can look at the documentation of FileBackend.