Hello,
Thanks for the great question. In the Version 1 Themes, like Hydra, the header background image is the same on each post and page.
This feature is available, in a way, in Version 2 of the BoldGrid Theme Framework, which you can start using with the Crio Supertheme. However, I should note, that in the current version of Crio, the header background images are chosen randomly, not per-page. This is a great idea for a feature request.
I tested out the WP Display Header plugin both with the BoldGrid Hydra Theme, as well as the default WordPress theme Twenty Nineteen and in each case the plugin has a fatal error on activation. The plugin has not been updated in 2 years, and its author isn’t responding to the support forum, so I don’t recommend trying this one out.
We do appreciate the feedback, and we’ll look at adding the capability to set a different header image per page in future releases.