WooCommerce is capable of showing category as well as products on its shop page and this is configurable from settings available in admin side but I can totally imagine running into the situation that categories don’t have their images uploaded and they will end up showing placeholder/default image.
A quick solution would be to pull a product image from that category and use that image in place of the thumbnail, highly dependent on what sort of products you are selling on your store, but in case you fit the bill, here’s how to do so:
Install this as a plugin, you can download it. Activate the plugin and any product category will now pull image from a product under that category and show it. In case an image is uploaded for the product category or one already exists, that takes a higher priority and is used instead of trying to get one out of a product.