$(document).ready(function() {

$("ul#topnav li").hover(function() { //Hover over event on list item
$(this).css({ 'background' : '#9999cc url(topnav_bottom_bg.jpg) repeat-x'}); //Add background color + image on hovered list item
$(this).find("span").show(); //Show the subnav
} , function() { //on hover out...
$(this).css({ 'background' : 'none'}); //Ditch the background
$(this).find("span").hide(); //Hide the subnav
});

});