Thank you for your question on preventing abckground images on all pages. It appears you are using BoldGrid. With BoldGrid, the Customizer is the configuration area for the theme, so any settings set there are theme wide. This includes the background image as that is not page-based.
The page content is edited in a different area (Page Editor) and you can add images there, but these will not override the main image in the theme area.
Thank you for your question on preventing abckground images on all pages. It appears you are using BoldGrid. With BoldGrid, the Customizer is the configuration area for the theme, so any settings set there are theme wide. This includes the background image as that is not page-based.
The page content is edited in a different area (Page Editor) and you can add images there, but these will not override the main image in the theme area.
Kindest Regards,
Scott M