wc-change-placeholder-image.php

/**
 * Change the placeholder image.
 */
function wb_woocommerce_placeholder_img_src( $src ) {
	$upload_dir = wp_upload_dir();
	$uploads    = untrailingslashit( $upload_dir['baseurl'] );
	// replace with path to your image.
	$src = $uploads . '/2012/07/thumb1.jpg';

	return $src;
}
add_filter( 'woocommerce_placeholder_img_src', 'wb_woocommerce_placeholder_img_src' );