Image cropping is only available to your customers if the Direct Fulfilment setting "Allow customer cropping" (available in Store > Settings > Direct Fulfilment Tab) is turned on. Otherwise, the decision about the image crop is left to you, the photographer. However, if this setting is turned on, this does not become a mandatory requirement for your customer's checkout process. The customer will still be able to place an order regardless of whether or not they have chosen a crop for their images.
If the customer does choose a crop, you will be able to view and adjust what they have chosen as the crop through your admin interface when viewing the order.
If they have not set a crop, you will be prompted to choose one before the order can proceed.
There is also a setting "Allow Auto Cropping" (also available un Store > Settings > Direct Fulfilment Tab) which allows orders to be automatically cropped if the image aspect ratio matches the required aspect ratio of the product. This allows for streamlined order management if you have also chosen to not check direct fulfilment orders (by choosing "off" for the setting "Check Orders" which is also found under the Store > Settings > Direct Fulfilment Tab of the admin interface).