Unique Wordpress Headers

Hi there,
I followed a WPMU article to have a custom unique header image on pages, categories etc.. but I'm struggling to get it to work.
This is the article: https://premium.wpmudev.org/blog/unique-wordpress-headers/
I installed the plugins and have uploaded a header to this page https://mumsbusiness.club/freebies/, however the header image doesn't display.
Can you advise where the problem may be?