This thread may help you out.
To copy and re-explain the relevant part, the key bit of CSS you'll be looking for is thus:
background: url(../images/image.png) no-repeat center;
That assumes the following file/folder structure, folders marked with square brackets:
So, the ../ tells the path to go "back one", starting from the file location it's triggered from - in this case, the story's html. so it goes BACK one level, then goes INTO the images folder (images/) and then selects image.png from that folder's contents. If you wanted to refer to say, a music file in some other scenario, the path would work thusly.
And assumes the following folder structure:
- EDIT -
I might note that if all your project files were in the same folder location, you shouldn't need the .../foldername/ part. But I would encourage you as part of good project management practice to sort files into appropriate folders!