if (document.images) {

   firm_off = new Image();
   firm_off.src = "images/nav_firm-off.gif";
   firm_on = new Image();
   firm_on.src = "images/nav_firm-on.gif";

   design_off = new Image();
   design_off.src = "images/nav_design-off.gif";
   design_on = new Image();
   design_on.src = "images/nav_design-on.gif";
   
   bios_off = new Image();
   bios_off.src = "images/nav_bios-off.gif";
   bios_on = new Image();
   bios_on.src = "images/nav_bios-on.gif";
   
   pdf_off = new Image();
   pdf_off.src = "images/nav_pdf-off.gif";
   pdf_on = new Image();
   pdf_on.src = "images/nav_pdf-on.gif";  
   
      
}

function imgOn(imgName) {
	if (document.images) {	
		document[imgName].src = eval(imgName + "_on.src")
	}
}

function imgOff(imgName) {
	if (document.images) {
   		document[imgName].src = eval(imgName + "_off.src")
   	}
}