The board allows to prevent that guests can't see external links and not properly pictures.
It means that when a picture is posted with a direct link, guests can't see the picture. When the picture is posted with a URL to a preview and is not directly linked, guests can not click the picture to see the preview page but they can see the picture although not working as a link.
For example, if you try to click on your pictures as a member, you'll be redirected to the page preview where it is hosted. As a guest, you can see your own picture but you cannot link on it.
If you don't want that your pictures are visible to guests, you need to use the bbcode that links directly to the picture and not to the page preview.
Susanne, I suppose you can see Sauroyd and Daria's pictures as a guest. Try to visit today's active topics of Daria's Merlin horse or Sauroyd's Mojo farm collection and let me know.