Děkuji za odpověď, na mém fóru to bohužel nefunguje.. Jedná se o fórum, které je součástí CMS PHP-Fusion (čili taková slabší odrůda phpBB) v 7.01.06
Nicméně, zdroj pro ten BBkód je takový:

if (!function_exists("img_bbcode_callback")) {
	function img_bbcode_callback($matches) {
		if (substr($matches[3], -1, 1) != "/") {
			return "<span style='display: block; width: 625px; max-height: 120px; overflow: auto;' class='forum-img-wrapper'><img src='".$matches[1].str_replace(array("?","&amp;","&","="), "", $matches[3]).$matches[4]."' alt='".$matches[3].$matches[4]."' style='border:0px' class='forum-img' /></span>";
		} else {
			return $matches[0];
		}
	}
}

$text = preg_replace_callback("#\[img\]((http|ftp|https|ftps)://)(.*?)(\.(jpg|jpeg|gif|png|JPG|JPEG|GIF|PNG))\[/img\]#si", "img_bbcode_callback", $text);
?>

$__BBCODE__[] = 
array(
"description"		=>	$locale['bb_img_description'],
"value"			=>	"img",
"bbcode_start"		=>	"[img]",
"bbcode_end"		=>	"[/img]",
"usage"			=>	"[img]".$locale['bb_img_usage']."[/img]"
);
?>