Choose a header image, selected from existing uploaded and default headers, or provide an array of uploaded header data (either new, or from media library).
(mixed) (Required) Which header image to select. Allows for values of 'random-default-image', for randomly cycling among the default images; 'random-uploaded-image', for randomly cycling among the uploaded images; the key of a default image registered for that theme; and the key of an image uploaded for that theme (the attachment ID of the image). Or an array of arguments: attachment_id, url, width, height. All are required.