I am afraid that I don’t have an example.

I will try and elaborate to make it more clear with what I am trying to achieve.

I am using the twenty fifteen theme and have included an image in the sidebar header.

As the user scrolls, I would like the image to scroll slowly behind the menu.

So I would like the menu to remain static and for the image behind it in the sidebar to slowly move as the user scrolls.

I would like the sidebar image to scroll at a slower pace than the main body of the site of possible.

I hope this makes it more clear with what I am trying to achieve.