//JMC => https://kwight.ca/2012/12/02/adding-a-logo-uploader-to-your-wordpress-site-with-the-theme-customizer/

function themeslug_theme_customizer( $wp_customize ) {
$wp_customize->add_section( 'themeslug_logo_section' , array(
    'title'       => __( 'Header Background image', 'themeslug' ),
    'priority'    => 30,
    'description' => 'Upload an image to be the background of the header section. Suggested size is 1500px wide by 250px tall',
) );

$wp_customize->add_setting( 'themeslug_logo' );

$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'themeslug_logo', array(
    'label'    => __( 'Logo', 'themeslug' ),
    'section'  => 'themeslug_logo_section',
    'settings' => 'themeslug_logo',
) ) );

add_theme_support( 'themeslug_logo', size );

}
add_action( 'customize_register', 'themeslug_theme_customizer' );