A header image in WordPress is a theme feature. Many WordPress free and premium themes comes with support for header image.
You can upload your own custom header image from the Appearance » Customize. WordPress will let you know the size requirements for the header image.
If you upload an image of the exact size, then it will be used as is. However, if you upload an image of a larger or different size, then you will be prompted to crop the image to fit the header image size.