Since upgrade to 1.8x I've problem with adding pictures to posts. It is strange as old posts can display number of pictures in one post and if I copy source code of old posts to the new one I got view of the name of the picture but no picture at all. It looks like in the attachment.
Please update wpForo to latest 1.8.2 version.
And please leave some URL to that page. Also, make sure you don't use an old Advanced Attachments Addon.