A "header image" refers to the picture or graphic that is displayed at the top of a webpage or document. It is often used to visually represent the content or purpose of the page. Full definition
For instance, some readers might see a different header image for a story than others. (entrepreneur.com)
Public events will now have larger header images, and will present more information about the location and event, such as an artist's profile or photos of the venue. (thenextweb.com)
We'll work with you on choosing colors and fonts, and will create a custom header image specifically designed to reflect you and your writing. (indieauthorservices.com)