1. Choose an image that you'd like to use for your background 

2. Next got to edit story 

3. Scroll down to add images

4. Then click add images. There are two options 

1) Main image 

2) Background image

5. To add images you must first upload them to a website that stores your pictures such as Imgur or Photobucket. If you haven't got an account then register one. Its simple and easy to use. Once you've registered your account click on your image. 

6. Click on direct link than paste it into the background image section

I hope that helped you! 

- Cara