You can set different background images for different pages through CSS.
First, find the unique class for each page or post. You can do that by looking at the body tag in the browser’s source code for each page or post.
For example, page-id-268 might be one unique page ID class, and you can target its background image like this:
.page-id-268 {
background-image: url('https://path/to/your/image.png') !important;
}
Change the image URL to match your particular image path.
You can add as many images as you like directly in your Media Library.
Don’t edit the theme directly or your changes will be overwritten the next time you update the theme.
An easy way to add custom CSS like this is to install the Jetpack plugin and activate the Custom CSS module. You could also install a standalone custom CSS plugin, or create a child theme.
Let me know how it goes!