PreImage1= new Image();
PreImage1.src = "img/nav1-bio-ovr.jpg";
PreImage2 = new Image();
PreImage2.src = "img/nav2-books-ovr.jpg";
PreImage3 = new Image();
PreImage3.src = "img/nav3-students-ovr.jpg";
PreImage4 = new Image();
PreImage4.src = "img/nav4-teachers-ovr.jpg";
PreImage5 = new Image();
PreImage5.src = "img/nav5-links-ovr.jpg";


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