Addition: I have had an idea/workaround on it... You can modify [img] or [imgs] codes so they allow only images loaded from your own forums URL. As smileys are placed on your local server, they should work (as well as it would work for any images placed on your own server).
Example (enCodeBB function of bb_codes.php):
/* [IMGS] tag code - with fixed width and ALT */
$pattern[]="#\[imgs\](http://".$GLOBALS['main_url']."/([^<> \n\r\[\]&]+?)\.?(gif|jpg|jpeg|png)?)\[/imgs\]#i";
/* Old [IMG] tag code - without fixed width - this one is used for smilies */
$pattern[]="#\[img\](http://".$GLOBALS['main_url']."([^<> \n\r\[\]&]+?)\.?(gif|jpg|jpeg|png)?)\[/img\]#i";