If you would like to allow your clients to crop the images that they add to their shopping cart you can follow these steps. Head to Selling » Selling Preferences and scroll to the Cart & Checkout section. There you can choose one of the three options for Image Cropping Options. Cropping is Not Permitted, Cropping is Optional and Cropping is Required.
Cropping is Not Permitted will not give the option for your client to crop their images.
Cropping is Optional will allow your client to crop the images but it isn't required.
Cropping is Required will request that the client crop the images before they can proceed with checking out.