If you choose to approve images before they are processed, then you can upload or choose any image you like in place of the one the customer chose. This means that you could choose a different crop or aspect ratio if you so desire. In other words, while you cannot alter the product the customer chooses to purchase, it is in your full control what appears on that product.
Of course, don't forget that with this power comes a responsibility to not misrepresent the product that your customer choses to purchase. If you aren't going to honor your customers' cropping choices, it may be better to disallow cropping at the outset.
--xsdg