The variable you are interested in is $vbulletin->options['maximages'] You may be able to create a plugin which sets that variable to '0' (meaning it's disabled) for admins. Perhaps try the global_start location with something along the lines of:
Code:
if (is_member_of($vbulletin->userinfo, 6))
{
$vbulletin->options['maximages'] = 0;
}