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.