That's because the URL is:
www.cornsnakes.com/forums/attachment.php?attachmentid=11708&stc=1
Putting a URL inside an image tag will generally work, as long as the file is something dot jpeg or something dot gif, etc. But if the image is one of the forum attachments, it won't do it, because what's actually being linked is a something dot php file, and not an image file.
I put them in the other thread using the html "img" tag (inside the <> instead of the [] brackets) like this:
[HTML]<img src=http://www.cornsnakes.com/forums/attachment.php?attachmentid=11708&stc=1>[/HTML]