Why is my correctly sized header image being cropped?

I have created header images following the WordPress criteria of 1000 x 288 and 25k max. When I upload these images, they usually appear cropped into square shape instead of their original rectangular form. When I go to the header page, these images do not show up as potential random header images, either in square shape or in the original rectangular shape.

How do I upload images 1000 x 288 images so that they will remain those dimensions after uploading — and how do I randomly use those images as my header image?

Thanks!