pic1= new Image(); 
pic1.src="img/nav-1-over.gif"; 
pic2= new Image(); 
pic2.src="img/nav-2-over.gif"; 
pic3= new Image(); 
pic3.src="img/nav-3-over.gif"; 
pic4= new Image(); 
pic4.src="img/nav-4-over.gif"; 
pic5= new Image(); 
pic5.src="img/nav-5-over.gif"; 
pic6= new Image(); 
pic6.src="img/nav-6-over.gif"; 

function swapOver(src) {
  document.getElementById("nav-" + src).src = "img/nav-" + src + "-over.gif";
}
function swapOut(src) {
  document.getElementById("nav-" + src).src = "img/nav-" + src + ".gif";
}