Comments

Are you using the layout maker or editing the css code itself? 

If it's the code, you can add the link to your background image in this section:

body, body.profile-page {

  background-image: url("link to your picture goes here") !important;

Using the layout maker helps & you can see a preview of how a profile would look using the layout. There are other settings that change how your background image looks so it's best to play around with the code or layout maker to see what you like the most. I hope this helps bc I'm not an expert at all, but I've had fun making layouts here.