okay = false;  
var preload = new Array ( 
 "images/home_on.gif",   "images/home_off.gif",
 "images/start_on.gif",  "images/start_off.gif",
 "images/letter_on.gif", "images/letter_off.gif"
 );

// find out what browser this is
with(navigator) {
  code = appCodeName; 
  app = appName; 
  version = appVersion; 
  iver = parseInt(version); 
  ua = userAgent;
  }

// this will work in "Mozilla" 3+ (includes MSIE 4)
if ( code == "Mozilla" && iver >= 3 )  okay = true;
else { okay = false; }

if(okay) {
  // compile the RegExp because we use it a lot
  var re = new RegExp();
  re.compile("[\\/.:\\-\\s]", "g");

  // preload the images
  for (var i = 0; i < preload.length; i++) {
    i_preload(preload[i]);
    }
  var preloaded = true
  }

// take a filename and make a legal variable name from it
function iname (img) 
{
var s = img.replace(re, "_");
return s;
}

// preload the images
function i_preload(img)
{
if(img) {
  var imgn = iname(img);
  eval(imgn + " = new Image()");
  eval(imgn + ".src = '" + img + "'");
  }
return true;
}

// swap entry function
function swap (name, image) 
{
if(!okay) return true; // just leave unless okay

// don't try to do this before the preloading is finished
if(preloaded) document.images[name].src = image;
return true;
}