This only seems to be with Firefox; Chrome and IE8 are OK.
See this thread here: http://xenforo.com/community/threads/test-of-new-attachment-buttons.1399/
When I first load that page, if I click on the large embedded image nothing happens.
If I refresh the page using F5 then it enlarges when I click on it.
It seems that the first time you load the page it doesn't work but it does after a refresh.