To change profile pic, go to "setting" menu, it's in the upper right hand corner.
Or you can access it by go to your profile page, and click on the drop down menu profile>>update profile.
Scroll down until you find "display picture" field, paste the image url (the link that ended with .jpg or .gif), then choose save profile.

For background pic and chapter pic: in edit story, under the foreword box there's a drop down menu "add images". Click it and paste the image url to the "main image" or "background image" as you want.
You can make each chapter having different pic, by editing each chapter through the same menu.