I need help adding an image behind my page links in my header

Lets start with I'm using square space CSS code injector. I am trying to have some images placed behind the text links that read (menu, what we sellin', wholesale) With my background image it is hard to see the links. I would like to add small wooden boards as the image behind them. Appreciate any help .

My website: www.slurpnsnack.com My website

edit: I do not want to add a background image to the header itself. I am wondering if there is a way to attach the picture itself to behind the link, or just how to go about placing my own links there.