STEP 1: To upload a new header image for your site, first choose an image, and size the part you wish to use to 187 x 1000 pixels. If you’re not sure how to do this, perhaps ask a photoshop guru. Large single images that are not too busy work well.
STEP 2: Then sign in to your site, and from the dashboard select Appearance > Header
STEP 3: Beside ‘upload image’, click Choose File and upload your image. You can select from the images uploaded previously, or have a randome one from this set displayed on every page.