var imagePrefix = "img/btn_";

function preload(){
	var newImages = new Array(	/* Button names */
	'home',
	'products',
	'mission',
	'contact'
	);

	for (i = 0; i < newImages.length; i++){  /* here walk through images array and preload */	
		newImage = new Image();
		eval("newImage.src = '" + imagePrefix + newImages[i] + "_on.gif';");
	}
}

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

function imgOff(imgName) {
	if (document.images) {    
		eval("document.images[imgName].src = '" + imagePrefix + imgName + ".gif'");
  }
}