You’ll need to use CSS.
If you have our Simple CSS plugin (https://wordpress.org/plugins/simple-css/), you can add your CSS into the “Simple CSS” metabox while editing the specific page:
body {
background-image: url( URL TO YOUR IMAGE );
}
Since there’s no page to edit for categories, you would have to add some global CSS in “Appearance > Simple CSS” and target the catgeory with the ID.
For example:
body.category-1 {
background-image: url( URL TO YOUR IMAGE );
}
1 being the ID of the category.