Create a custom page as follows:

1. Create an appropriate image
2. create an article and put the image in to it (and any other text required)
   1. dont forget to put a link back to othe home page, otherwise prople will really be lost in space
3. create a hidden menu.....    give it a type of say, "hidden-menu"
4. create a menu item under this hidden menu, linking to the article
5. change the error.php file in the template and/or system directory, and add the following code in an appropriate spot (where "404-page-not-found" is the hidden menu item):
   1. generally put this code just before the existing default error stuff, which starts with:    ?php echo JText::_('YOU_MAY_NOT_BE_ABLE_TO_VISIT_THIS_PAGE_BECAUSE_OF')....  etc

         <?php
            /*       MBL  Error page code    */
                if (($this->error->getCode()) == '404') {
                header('Location: http://mobiusmbl.com.au/404-page-not-found');
                exit;
                }
            ?>