add_action("gform_user_registered", "image_author", 10, 4);
function image_author($user_id, $config, $entry, $user_pass) 
{
$post_id = $entry["post_id"];

$args = array(
\'post_parent\' => $post_id,
\'post_type\' => \'attachment\',
\'post_mime_type\' => \'image\'
);

$attachments = get_posts($args);
if($attachments) :
    foreach ($attachments as $attachment) : setup_postdata($attachment);
       $the_post = array();
       $the_post[\'ID\'] = $attachment->ID;
       $the_post[\'post_author\'] = $user_id;

       wp_update_post( $the_post );

   endforeach;
endif;    

}