Unless the uploaded image is proportional to the req'd dimensions, it could never be done reliably. Images that people typically have are relatively "square" while this banner needs to be what, 5-6x wider than tall ?
It could work like Facebook, Linkedin, etc where it allows any size image and then forces the user to position it within the cropped size. Even that doesn't work well.... Look at a lot of people's banner image on Facebook where all you see is their family's mid-section 'cause their legs and heads get cropped off.