Surprisingly WooCommerce Wordpress ECommerce doesn't directly provide the option to replace the Placeholder Image/Temporary Product Image/Default Product Image, so if you are stuck here's how to do it. Add these snippets to your Child Theme functions.php page:-
Woo Commerce Custom Placeholder Image On Single Product Page
Woo Commerce Custom Placeholder Image On Archive & Category page
There are a few other methods out there, but we prefer this approach as you can easily specify the URL to your placeholder image.