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/*
* goes in theme functions.php or a custom plugin. Replace the image filename/path with your own :)
*
**/
add_action( 'init', 'custom_thumbnail' );
function custom_thumbnail() {
  add_filter('woocommerce_placeholder_img_src', 'custom_woocommerce_placeholder_img_src');
   
    function custom_woocommerce_placeholder_img_src( $src ) {
    $upload_dir = wp_upload_dir();
    $uploads = untrailingslashit( $upload_dir['baseurl'] );
    $src = $uploads . '/2012/07/thumb1.jpg';
     
    return $src;
  }
}