In our our themes default Woocommerce functionality is used and mentioned part of functionality is not an exception.If you want to replace default “image not found” for products on archive,single pages etc
just follow wp-content/plugins/woocommerce/assets/images/placeholder.png straight on your server and replace current placeholder.png image with needed one.Image name should be the same (placeholder.png)
Then refresh your site in browser with ctrl+shift+R