[Lesane]

Try this:

Open admin/functions.php, look around line 884 for this line:

PHP Code:

      $bbcode = preg_replace("/\[img\](\r\n|\r|\n)*((http|https):\/\/([^;<>\(\)\"".iif($allowdynimg,"","!\?\&")."]+)|[a-z0-9\/\\\._\- ]+)\[\/img\]/siU", "<img src=\"\\2\" border=\"0\" alt=\"\">", $bbcode); 


Replace that line with:

PHP Code:

 if($bbuserinfo[userid]==0) {
      $bbcode = preg_replace("|\[img\](.*)\[/img\]|i", "<img src=\"http://www.forumurl.com/youllneedtoregister.jpg\" border=\"0\" alt=\"\">", $bbcode);
      } else {    
      $bbcode = preg_replace("/\[img\](\r\n|\r|\n)*((http|https):\/\/([^;<>\(\)\"".iif($allowdynimg,"","!\?\&")."]+)|[a-z0-9\/\\\._\- ]+)\[\/img\]/siU", "<img src=\"\\2\" border=\"0\" alt=\"\">", $bbcode);
      } 


Change 'http://www.forumurl.com/youllneedtoregister.jpg' into the correct url for the image to display for guests.

That should do the job.