I just logged out and had a look. Members can see attached images, but non-members can't. Non-members also can't see photos in people's profiles (which makes sense).
Location: Spain, residing in the USA, United States
Posts: 1,518
That was one of the main reasons I ended signing up to the forum about a year ago, I could not open the thumbnails or the picture links unless I logged in.
Non-member used to be able to view everything that members could. The only difference was that non-members could not make posts or send private messages.
However, several months ago when we started to experience our bandwidth problems, our settings were changed to conserve bandwidth by restricting access. Now only members can view attachments. For non-members, posts are blank or only have the text so that they know there are pictures but they can't see them.