Thanks for sharing the screenshot.
As per my understanding from screenshot, you are using correct approach, background image will get replace by clicking on Replace on left side.
But if this is not working then you have to check with Hubspot support by sharing the name of the particular theme that you are using to create the page.
Another way is that you can share the web URL/Publish URL of page with me so I can share the CSS that you can add on particuar page to change the background image.
Hope you will understand.
Thanks
Niharika Goyal