I was trying to do the same thing, and I spent many hours searching and trying different things different forums offered, like this one too.
The following worked the best for me.
https://forum.bytesforall.com/showthread.php?t=9525
Here is my testing site.
https://tiffany.hyukit.com/?page_id=220
I wanted the About page and the Services page to have an image background in the actual content section. But not on the other pages.
Here is the code I put in CSS
body.page-id-220 div#content-area {
background-image:url(‘https://tiffany.hyukit.com/wp-content/uploads/2013/03/About-Page-Background1.png’);
}
I found out what the page name was from mozilla view source.