Hey there Sjoerd,
Hope you’re well today!
You could achieve that with some custom CSS. First you can set background image in Appearance >> Customize for your home page and all other pages where you will not change the background. For other specific pages you’ll have to target page ID which you can see by using developer tools and inspecting the page. Here is an example of one of my pages https://screencast.com/t/MY2DlTTlQ.
When done with that try adding the following CSS code in the style.css file of your child theme or add it in your site using the following plugin:
https://www.ads-software.com/plugins/simple-custom-css
body.page-id-163.custom-background {
background-image: url('your_image_URL');
}
This should change background image on that specific page. For other pages simply repeat the process just change the “page-id-163” part with the correct page id.
If this doesn’t work could you please post link to your site once you add the code so I can take a look?
Hope this helps ??
Best regards,
Bojan