var path = "images/"
var extension = "jpg"
var on = "_on." + extension
var off = "_off." + extension

function preload()
{ for (var i=0; i<buttons.length; i++)
  { var button = new Image;
    button.src = path + buttons[i] + on;
  }
}

function switchImg(img)
{ if (document.images[img].src.indexOf(on) != -1)
    document.images[img].src = path + img + off;
  else document.images[img].src = path + img + on;
}

preload();