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if ( !function_exists('custom_gravatars') ) {
    function custom_gravatars( $avatar_defaults ) {
        $myavatar = get_template_directory_uri() . '/images/custom-avatar.png';
        $avatar_defaults[$myavatar] = 'people';
        return $avatar_defaults;
    }
add_filter( 'avatar_defaults', 'custom_gravatars' );
}