How to scroll background pictures on the homepage as you scroll down the page

Hi guys,

I am getting to grips with the ample theme, would just like to know how do you add a background image but then see bits of the background image appear in sections of your homepage as you scroll down as is shown in the example of what the theme looks like.

The footer is appearing above it for some reason and i can only see about one cm of the background image after the footer if that makes sense.

Any help and advice much appreciated in advance.

Thank you