I know you've successfully attached pictures to messages on this BBS before, so I know it's not likely to be anything you're doing wrong.

Quick checks: The filename, or folder name, of the picture you're attaching isn't somehow unusual, or unusually long, or contains odd characters, does it? The file type is ordinary JPG, right? The file size isn't unusually large, is it?

Here, I'll try attaching a little JPG really quick and see if it works for me...