I grep'ed through the source code and probably its:

Code:
./includes/class_bbcode_alt.php:                    'src'          => $vbulletin->options['bburl'] . '/attachment.php?attachmentid=' . $attachmentid . '&stc=1',
So $vbulletin->options['bburl'] is still pointing to http://, but couldn't find yet where it is initialized. I've asked in vBulletin forum, I hope they gonna answer, shouldn't be so hard for those people.