// W PLIKU header.php ZMIENIASZ TEN KAWAŁEK KODU:

<hgroup>
	<h1 class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></h1>
	<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
</hgroup>

// NA TEN:

<?php if ( get_theme_mod( 'twentytwelve_logo' ) ) : ?>
    <div class='site-logo'>
        <a href='<?php echo esc_url( home_url( '/' ) ); ?>' title='<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>' rel='home'><img src='<?php echo esc_url( get_theme_mod( 'twentytwelve_logo' ) ); ?>' alt='<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>'></a>
    </div>
<?php else : ?>
	<hgroup>
		<h1 class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></h1>
		<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
	</hgroup>
<?php endif; ?>

//POTEM W functions.php DODAJESZ TAKI KAWAŁEK:

function twentytwelve_theme_customizer( $wp_customize ) {
 $wp_customize->add_section( 'twentytwelve_logo_section' , array(
  'title'  => __( 'Logo', 'twentytwelve' ),
  'priority' => 30,
  'description' => 'Upload a logo to replace the default site name and description in the header',
) );
$wp_customize->add_setting( 'twentytwelve_logo' );
$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'twentytwelve_logo', array(
 'label' => __( 'Logo', 'twentytwelve' ),
 'section'  => 'twentytwelve_logo_section',
 'settings' => 'twentytwelve_logo',
) ) );
}
add_action('customize_register', 'twentytwelve_theme_customizer');