If you are running the latest version of the theme it is possible to assign a background image to any sidebar. Under Appearance → Theme Options → Sidebars select “Edit Sidebar” next to the name of the sidebar you want to assign a background image to and under background options select the background image.
I assume you put the slider in the sidebar location called “Full screen”. Right below this sidebar you have another sidebar location called “Header”. If this is confusing for you maybe this article will help.