WP_Customize_Image_Control and selecting from library?

Question

I am adding some theme options, one of which is an image option.

 $wp_customize->add_control(
            new WP_Customize_Image_Control(
                $wp_customize,
                'theme_ppr_options[portfolio_photo]',
                array(
                    'label'      => __( 'Upload a portfolio', 'ppr' ),
                    'section'    => 'mytheme_options',
                    'settings'   => 'theme_ppr_options[portfolio_photo]',
                    'context'    => 'your_setting_context' 
                )
            )
        );

But i notice i am not able to select an image from the library, how can this be enabled??

regards