I don't seem to be able to see any images posted in forum threads; I just see a grey box with 'Attached Thumbnails' written in it (which is not click-able.)
I have 'show images' enabled in my profile settings.
EDIT: I since found this thread which seems to indicate this is a site-wide issue:
http://forums.cgarchitect.com/71256-cant-view-attached-images-while-logged.html