Any images that you place in the folder:
/public_html/wp-content/themes/atahualpa/images/header
will display in the header.
If there is more than one image in the folder they will display randomly each time a web page is opened
It’s recommended that the images be 1300 pixels wide to cater for most screens. On this website they are all 150 pixels high but not all 1300 pixels wide.