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/placeholder.png';
    return $src;
}