Content of header image moving up and down as browser width changes

I want an image between the primary and secondary navigations, so I uploaded an image to / Background images / header.

The problem is that the content of the image moves up or down depending on how far out I pull the browser window, and sometimes the content of the image is being lost behind the main navigation.

When I inspect it I see that it is div class "inside-header" and its dimensions vary depending on how far out I pull the browser window. The vertical distance stays the same but the horizontal dimension changes, eg 1352x164 or 913x164.

I don't understand what the choices (attachment. Fixed, local, inherit) mean, but I have tried various settings and uploaded the image at various sizes and aspect ratios, and I cannot get the content of the image to remain at the same visibility whatever the browser width.

I watched the video on the header element but the page hero seems to be about featured images in posts and the Site Header section doesn't have an option for an image to be uploaded or chosen, so that doesn't seem to be what I am after, and I am stuck.

Thanks