Oh I think I also misunderstood the problem you were having. When an image attachment is included in an article by an admin, and a regular user clicks that image to view in a new window, the user receives a permission error?
If that's the case check Forums & Moderators > Forum Permissions > [forum name] > Registered Users, and make sure "Can Download Attachments" is set to Yes. If it is, you may simply need to rebuild the permissions cache, so just click Save.