Add a Image Field to WP Customizer

WP Customizer - Image Control

$wp_customize->add_setting( 'themeslug_media_setting_id', array(
  'sanitize_callback' => 'absint',
  'validate_callback' => 'themeslug_validate_image,
) );

$wp_customize->add_control(
  new WP_Customize_Media_Control( $wp_customize, 'themeslug_media_setting_id', array(
    'label' => __( 'Custom Core Media Setting' ),
    'section' => 'custom_section', // Add a default or your own section
    'mime_type' => 'image',
)	)	);

function themeslug_validate_image( $validity, $value ) {

  // Get the url of the image
  $image = wp_get_attachment_image_src( $value )[0];

  /*
  * Array of valid image file types.
  *
  * The array includes image mime types that are included in wp_get_mime_types()
  */
  $mimes = array(
      'jpg|jpeg|jpe' => 'image/jpeg',
      'gif'          => 'image/gif',
      'png'          => 'image/png',
      'bmp'          => 'image/bmp',
      'tif|tiff'     => 'image/tiff',
      'ico'          => 'image/x-icon'
  );
  // Return an array with file extension and mime_type.
  $file = wp_check_filetype( $image, $mimes );
  
  if( !$value ) {
    // If no image has been chosen, instruct user to choose an image
    $validity->add( 'required', __( 'Please choose an image' ) );
  } elseif ( !$file['ext'] ) {
    // If a valid image file extension is not found, instruct user to choose appropriate image
    $validity->add( 'not_valid', __( 'Please choose a valid image type' ) );
  }
  return $validity;
}