var imagesPath = 'Images/';

function PreloadImages() {
  for (var i = 0; i < arguments.length; i++) {
    var image = new Image();
    image.src = imagesPath + arguments[i];
  }
}

function SwapImage(image, imageSrc) {
  if (image != null) {
    image.src = imagesPath + imageSrc;
  }
}

PreloadImages(
  'Nav-Home-Over.jpg',
  'Nav-Portfolio-Over.jpg', 
  'Nav-Expertise-Over.jpg',
  'Nav-CV-Over.jpg',
  'Nav-Contact-Over.jpg'
  );