Maybe the method you’re using to display the banner is using a specific image size. Usually wordpress can use a defined (or not) image size to display a media item, but if you check the media item, you’ll be able to see the original upload.
Can you provide some code where you’re displaying the banner?
You should check out the add_image_size, get_the_post_thumbnail methods and also if you should change the method to display the image.