BuddyPress.org

Sorted.

You just need to insert !important in the CSS code just before the ; and it over-rides the original

I created the child page as per https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/

added an img folder and new image in that folder.

in the css I put

#header {

background: url(full http url to new image.jpg ) !important ;

}

Thank you to both of you who pointed me in the right direction.