I generally recommend to people to crop and size their own images before uploading because doing that first gives a lot more flexibility for a final image. I’m assuming you are referring to the WP Header feature? When you upload an image, it’s important to make sure it’s equal or larger than the size being used. When you upload it, the feature has it’s own cropping tool that you can move around the image to crop it, otherwise, I still say it’s better to prepare the images before you upload them.
Please note when using the cropping tool in the WP Header or even when you upload an image for a page or post, if your image is smaller than needed, WordPress does not make it bigger to fit.