if (document.images) {
  
  abouton = new Image();
  abouton.src = "img/bt_about_over.gif";
  aboutoff = new Image();
  aboutoff.src = "img/bt_about.gif";

  portfolioon = new Image();
  portfolioon.src = "img/bt_portfolio_over.gif";
  portfoliooff = new Image();
  portfoliooff.src = "img/bt_portfolio.gif";

  arton = new Image();
  arton.src = "img/bt_art_over.gif";
  artoff = new Image();
  artoff.src = "img/bt_art.gif";
  
}