Background Image Overlapping in mobile browsers in Parallax website

I m making a parallax website. The website works fine on desktop browser but when I view it on mobile browser the background image overlaps the foreground image while scrolling.

I am using stellar js for vertical scrolling nicescrollbar and below is the link for the code for movement of the background image: http://ift.tt/1Dowshl - I want that working for IOS also, so have removed the 'if' condition for disabling it for IOS.

Below is the link of the : http://ift.tt/1zMVTMs