Weebly background image issue

I am designing a website for a friend in Weebly, I tried to convince them to use wordpress but they really want Weebly.

Anyway, I'm pretty much done with the home page but for some reason when I pull it up on my phone, the background image jerks around as I scroll, I've tried everything I know how to do and I can't fix this.

Any help? http://ift.tt/2aNA7Ng

Thanks.