$wp_customize->add_setting( 'DD_1st_slide_background' );

$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'DD_1st_slide_background', array(
    'label'    => '1st Slide, background',
    'priority' => 1,
    'section'  => 'DD_slider_section',
    'settings' => 'DD_1st_slide_background',
) ) );