To change the header on a single site page (without changing any other pages), upload an image that is the same size or larger than the site header (1000 x 288 pixels). At the bottom right side menu on the edit page screen, make your new uploaded image the “Featured Image” for your page.
The featured image can also be used to select any image on your page or post to be featured in RSS readers that include images.
This image is 2337 x 1420 pixels. It is shown in its original aspect ratio above, and as auto-cropped by WordPress in the header.