Cropping warnings may appear in two places when you are checking out: in the confirmation screen and in the cart:

A cropping warning appears when you have cropped more than 20% of an image. This may be intentional; for instance, you may want to print only the face of the woman in this painting:

The cropping warning gives you a chance to amend your cropping. If you are satisfied with your cropping, simply approve your order in the Confirmation screen at the end of the checkout process. Your order cannot be changed after it has been placed.