Those images are dynamic in that they are generated by PHP code (or some other language). There are certain file suffix restrictions you can use for avatars .gif, .jpg etc but .php is not one of them for security reasons. In addition, the avatars are, as someone suggested, stored on the database in a binary format and a PHP image cannot be stored in such a way easily so you can't use them
In addition, avatars per forum is not an option.