When looking for ways to set permissions on a galley, I came across this:
In the Enterprise version of the SilverStripe CMS software, you are able to set permissions for the folders in which the files and images are located. By setting permissions, you can determine which users have access to the files and images in the folder, thereby controlling who can update, add and/or remove the contents.
Where is this Enterprise edition? Is this info out of date? How can I set permissions on the Gallery folders?
You can set permissions in the "Access" tab of the gallery "folder" in the Site Contents area.
This will protect some visiting http://www.yoursite.com/your-gallery-of-boats-and-ships/ page although it won't prevent someone knowing the specific URL of an image file, and giving that to someone else to view.
I believe the userhelp documentation referring to that enterprise edition is out of date. I've added [url=http://open.silverstripe.com/ticket/1811]a request to fix it[/url].