if (document.images) {
   on = new Array(9)
      on[1] = new Image(); on[1].src = "gfx/rollover.jpg";
      on[2] = new Image(); on[2].src = "gfx/roll-cats.jpg";
      on[3] = new Image(); on[3].src = "gfx/roll-dogs.jpg";
      on[4] = new Image(); on[4].src = "gfx/roll-birds.jpg";
      on[5] = new Image(); on[5].src = "gfx/roll-fish.jpg";
      on[6] = new Image(); on[6].src = "gfx/roll-reptiles.jpg";
      on[7] = new Image(); on[7].src = "gfx/roll-smallpets.jpg";
      on[8] = new Image(); on[8].src = "gfx/roll-about.jpg";
      on[9] = new Image(); on[9].src = "gfx/roll-contact.jpg";
}
function swap(buttonName,num) {
    if (document.images) {
      document[buttonName].src = on[num].src; }
}