The right solution depends on the particulars of your installation. I recommend temporarily giving the Everyone group Full permission to that directory, being sure to use the advanced option to re-apply the permissions on all child objects. That should guarantee access, and once you verify that, you can play around with specific account and groups until you've narrowed it down to the right one.
Just don't leave the Everyone group in charge of that directory - big security hole!
Roger Martin
Creator and Lead Developer of Gallery Server Pro