I have noticed that when the background image is set to "fixed" mode then it doesn't work on Iphone and Ipads. However, when it is set to "scrolling" mode then it works. I am using the latest version of SPPB.
While most web designers will try it on Android platforms and see it working perfectly, those with iPhones and iPads will see a poorly designed, unprofessional website. I am not sure if you're aware of it or not but this is a major bug and should be fixed immediately.
I discovered this while working with an European client and they were pretty unhappy about this.
Please look into this urgently