if(navigator.appVersion.charAt(0) >= 3) {
 var img = new Array();
 img[0] = new Image();  img[0].src = "images/btn_mail1.gif";
 img[1] = new Image();  img[1].src = "images/btn_mail2.gif";
 img[2] = new Image();  img[2].src = "images/btn_blog1.gif";
 img[3] = new Image();  img[3].src = "images/btn_blog2.gif";

}

function change_image(nam,num) {
  if(navigator.appVersion.charAt(0) >= 3) {
    document.images[nam].src = img[num].src;
  }
}