The placeholder image can now be set at: WooCommerce > Settings > Products as of version 3.5x.

WooCommerce 3.4x or earlier

Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.

/**
 * Change the placeholder image
 */
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'] );
	// replace with path to your image
	$src = $uploads . '/2012/07/thumb1.jpg';
	 
	return $src;
}