Adding image background to main navigation menu only when it scrolls as sticky

Hi

Thanks much for answering all my questions…..

1) I am working with the navigation bar, is it possible to add an image instead of using color scheme. For example with my website I would like for when I scroll the nav image is consistent with the header background image as its custom to my design.
I have created the image for that but was not sure on how to and if it can be done

2)The back to top—- how do I scale or change the icon/image?

http://www.carolinemarchionna.com/home-3/

Thank you