Option 2: Retina image control in the Wordpress Customizer

gistfile1.txt

/**
 * Retina image control.
 *
 * This solution involves creating a new control, implemented as the 
 * Sofa_Customizer_Retina_Image_Control class.
 * 
 * Using this control, we get a regular image control with a simple
 * checkbox underneath asking the user whether the image is retina-ready. 
 * This means that the user can just upload a single image and tick a box 
 * if it is correctly sized for retina screens (i.e. it's twice the size
 * of its regular display). 
 *
 * Under the hood, the Sofa_Customizer_Retina_Image_Control creates the 
 * non-retina version of the image if the checkbox is ticked. It then 
 * modifies the logo settings that are stored in the database with references
 * to the retina image's attachment ID, the URL of the non-retina version and
 * the dimensions of the non-retina version.
 */
$wp_customize->add_control( 
    new Sofa_Customizer_Retina_Image_Control( 
        $wp_customize, 
        'logo',
        array(
            'settings' => 'logo',
            'section'  => 'header',
            'label'    => __( 'Logo', 'crafted' ), 
            'priority' => 34
        ) 
    )
);