OK I can see the thumbnail again, and it's larger. But clicking on it still produces the error. What is supposed to happen is that a window pops up with the picture made much larger. After all, a thumbnail is just a sample - clicking is supposed to show the full size. So there's either a bad setting in the forum's admin page, or incorrect file permissions on the server.
Also, from the main page of this site there is no link to the forum (or if there is, it isn't obvious).
Also, from the main page of this site there is no link to the forum (or if there is, it isn't obvious).