You should tag your question with the twine version and the type and version of the twine story format you're using, since the answers will depend on them.
For an example, when using TweeGo and SugarCube 2 (any reasonably recent version), you can just put the background image file - let's name it "background.jpg" - into one of the directories you use to compile your story, then reference it in the CSS like this to put it as the story section's background image.
#story {
background-image: [img[background]];
}
The "background" part here comes from the file name. If your file was named "paper.jpg", you'd use "background-image: [img[paper]];", and so on.