Right now it is not possible to fetch page via API and access header image.

Lets say I want to render page and I call:

GET https://{instance-name}.atlassian.net/wiki/rest/api/content/{content-id}?expand=body.view,body.storage,body.editor2,body.editor,body.atlas_doc_format,body.dynamic,body.export_view,body.styled_view

There is no information about header image used on confluence page in the response.

Please consider adding information about header image added to page in content REST API response and allow to fetch header image, example:

GET https://api.media.atlassian.com/file/{header-image-id}/image

or include direct url to header image in response.