Image Behind Nav Bar

Hello,

I recently used the tutorial on this site on how to make a horizontal navigation bar, which worked nicely, but now I would like to put an image behind that menu bar. How do I do this?

Here is my image codes for my .css-

#navmenu ul {margin: 0; padding: 0; text-align:center;
list-style-type: none;list-style-image; }
#navmenu li {display: inline; }
#navmenu ul li a {text-decoration:none; margin: 4px;
padding: 5px 20px 5px 20px; color: dark yellow;
background:}
#navmenu ul li a:hover {color: purple;
background: }

Thanks!