if (document.images) {

var image1 = new Image();
image1.src = "images/youtube_off.png";
var image2 = new Image();
image2.src = "images/youtube_on.png";
var image3 = new Image();
image3.src = "images/facebook_off.png";
var image4 = new Image();
image4.src = "images/facebook_on.png";
var image5 = new Image();
image5.src = "images/twitter_off.png";
var image6 = new Image();
image6.src = "images/twitter_on.png";

}

function roll(img_name, img_src)
   {
   document[img_name].src = img_src;
   }