Site-wide Header Image

You can set the header image site-wide by adding the following code to your _config.yml

header: images/IMAGE-NAME.png

Per Page Header Image

You can also specify a different header image for any giver page by adding it to the yaml frontmatter for that page:

header: /images/IMAGE-NAME.png

(If you notice, this page has a custom header image from the rest of the site)