If these conditions are met:
- there are a category and its subforum, both are (the first category is thus a "subcategory").
- the guest or a given user cannot see that subforum, although that latter exists (due to permissions).
He or she can see the category on the index page (the category should be invisible, just like a category without any forum) and no forum image is display. I didn't detect that bug with viewforum.php (e.g. if you look inside a category with a such subcategory), only with index.php.
The bug seems to be not style-related, I encountered that not only on prosilver or other board styles using the forumlist_body.html template file.
I attached two screenshots below.
EDIT: the bugs also appears on index.php if there are three empty categories (a category located at the index, the second category located inside the first one and, finally, the third category located inside the second one).