Add a setting for links to images and whether or not mods allow them
Sometimes people get around the no image-posts rule by linking to images, or sometimes people need to link to an image in a circle that is for images but not for links.
Most image posts are to image files or to imgur or another common image host. We could treat these links as their own category of link, and allow mods to allow them or not allow them separately from the setting for links.