If you are interested in creating a custom header image for your website, I recommend using Canva

1. In Canva, start a new design, using a custom size. 

2. The size should be set to 1176 px x 310 px for a standard header. 

3. When the file is created title it something like "Website header" or "Homepage Header"

4. The color scheme you use for the design should match what you're using on the site theme.

5. When you're done, download the image as a .PNG

If you have multiple pages, it will download as a zipped folder that you will need to extract. 

6. While you are on the screen to edit your site, click on the "Image" button on the bottom left of your website header, click "Upload" then find your header image download.

7. You will need to add the image to each header for each page, it will not automatically put it on all pages.