A bit late to the party here, but I think this plug-in will do what you require. I have a header image on my main blog/home page but the plug-in gives the option to change the image header on a post page.
https://geek.hellyer.kiwi/plugins/unique-headers/
Hope this helps.
Paul
Thanks for your reply, Mirucon.
I’ll leave it as is and see if WP can sort out their issue.
Regards,
Paul
