The empty box that shows instead of the real/actual image is located at:
../wp-content/plugins/woocommerce/assets/images/placeholder.png
what one has to do is on the right of the 'edit product page' is select the 'featured image' (not the product gallery images, althoug this could be done also to show several images of same product). Once you update the 'edited product page' it should now replace the placeholder.png image with the right product image.
good luck, keep selling!