var type1_ = new Image;
var type2_ = new Image;
var type3_ = new Image;
type1_.src = "img/1_.jpg";
type2_.src = "img/2_.jpg";
type3_.src = "img/3_.jpg";

function typeOver(td, num) {
	td.style.backgroundImage = 'url(img/' + num + '_.jpg)';
}
function typeOut(td, num) {
	td.style.backgroundImage = 'url(img/' + num + '.jpg)';
}