Option 1: Retina image control in the Wordpress Customizer

gistfile1.txt

/**
 * Retina image control. 
 *
 * Essentially, you just create two separate image controls, one of which is for the regular version of the logo, 
 * the other for the retina version. 
 *
 * This is a simple solution with a few downsides: 
 *
 * 1. It can be confusing for users who may not understand what is required for a retina version.
 * 2. There is nothing enforcing the 2x sizing of the retina version. 
 */
$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'logo_url',
    array(
        'settings' => 'logo_url',
        'section'  => 'title_tagline',
        'label'    => __( 'Logo', 'my_theme' )
    ) )
);
$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'retina_logo_url',
    array(
        'settings' => 'retina_logo_url',
        'section'  => 'title_tagline',
        'label'    => __( 'Retina version of logo (2x)', 'my_theme' )
    ) )
);