It seems that the Image BB-Code doesn't work at all under vBulletin 3. Its version of attachment.php exits prematurely if the attachmentid isn't part of the URL and we get a 404 error. I don't know how in the past 4 years of this feature, no one reported that. Anyway, I have fixed it on your installation and made the appropriate changes to the next release.
Marking as Fixed for the next build.