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and copy your created code

Go to your website editor

Appearances > Editor > 404 template

Search the next part

 <div class="page-content"><p><?php _e( 'Nothing could be found at this location, maybe try a search?', 'cc2' ); ?></p>                            <?php get_search_form(); ?>

Paste your created picture code as follow

<div class="page-content"><p><?php _e( '<a href="http://workathomefuture.com"><img class="aligncenter size-full wp-image-4350" src="http://workathomefuture.com/wp-content/uploads/2015/07/404.jpg" alt="404" width="607" height="477" /></a>', 'cc2' ); ?></p>