You can restrict viewing two ways. You can set under Admin => Registration Options for guests to register to view images which means they will see the categories and thumbnails but not the medium or large images. Or you can go to Edit Categories in Admin and unselect guests from each category you edit the viewing permissinons and they will not be able to see any photos or categories you remove them from. It is always preferable to leave a cat or two viewable and restrict viewing of large images in combination to give them a teaser to register.
