Hello Matthew. My former school where I taught also uses Moodle as their website and Lightbox gallery for photos - www.olchs.lancs.sch.uk The galleries are added to a course with guest access and so the gallery itself allows anyone to see. What do you mean by unauthenticated users?
OK if you mean you want users just to be able to click on the gallery link and see the images (like here) then if you go to Site administration>Users>Permissions>User policies and tick "auto-log in guests" they should be able to see guest access pages/courses immediately without clcking to log in as guests, assuming such pages are open to guests already.
