Hi there,
simplest way is to use a Hook Element to add your custom styles:
https://docs.generatepress.com/article/hooks-element-overview/
Create a new hook
Add your CSS styles eg.
<style>
body {
background-image: url('full-url-to-the-image');
background-size: cover;
background-repeat: no-repeat;
}
</style>
Set the Hook to: wp_head
Set the Display Rules to the pages / posts or post category etc. for where you want it displayed.
Repeat the above for each different image group.