Customize WooCommerce placeholder image

add_filter( 'woocommerce_placeholder_img_src', 'psq47_custom_woocommerce_placeholder', 10 );

/*
 * Function to return new placeholder image URL.
*/
function psq47_custom_woocommerce_placeholder( $image_url ) {
  $image_url = 'http://wc23.co/wp-content/uploads/2015/08/placeholder.png';  // change this to the URL to your custom placeholder
  return $image_url;
}