This might not be strictly a bug, but misconfiguration of the server settings, however it might not be clear to a normal user.

The scenario:
Force server URL settings on: Yes
Script path: empty

I'm posting an image hosted on the same URL as the board, e.g.

[img]http://www.phpbb.cz/adm/images/icon_edit.gif[/img]

Posting produces this error:

[phpBB Debug] PHP Notice: in file /includes/message_parser.php on line 994: strpos() [function.strpos]: Empty delimiter.

Since in this function call:

strpos($url, $check_path) 

$check_path is the script_path in the ACP and is empty when calling the function.