If a buyer selects a standard size, your image will be center cropped to that size. There is no way for you to control the crop position right now. If you're not happy with your images being center cropped, then you should opt-out of cropping altogether.
When uploading, you can say you want standard sizing but they will be cropped if you do that, centrally. Unfortunately, you usually can't make your images fit into standard sizes without cropping the images.