Hi there,
Are you talking about the body background image?
If so CSS targetting the page ID is required.
For example, if we want to change the background image for Seilklettertechnik page, then we need this CSS:
body.page-id-150 {
background-image: url(https://URL-HERE);
}
and if we want to remove the background image for that page, then we need this CSS:
body.page-id-150 {
background-image: none
}
Adding CSS: https://docs.generatepress.com/article/adding-css/
Let me know if this helps :);