I think I did it!
Hi, to change the background click above your profile photo, it says "customize", from there you will have some options. On the first set select the "change the look & feel", then once on the next screen go down to the section called "Profile Wallpaper" there you use the "Background image" browse option. Load up the one you want, then go to the bottom, hit save and there you go! You may have to once you get back to the profile and you are viewing it, it refresh to get it to come up. Good luck!