images = new Array();
images[0]       = new Image();
images[0].src   = "images/n1_1.gif";
images[1]       = new Image();
images[1].src   = "images/n1_2.gif";
images[2]       = new Image();
images[2].src   = "images/n2_1.gif";
images[3]       = new Image();
images[3].src   = "images/n2_2.gif";
images[4]       = new Image();
images[4].src   = "images/n3_1.gif";
images[5]       = new Image();
images[5].src   = "images/n3_2.gif";
images[6]       = new Image();
images[6].src   = "images/n4_1.gif";
images[7]       = new Image();
images[7].src   = "images/n4_2.gif";
images[8]       = new Image();
images[8].src   = "images/n5_1.gif";
images[9]       = new Image();
images[9].src   = "images/n5_2.gif";

function swap(name,idx) {
        document.images[name].src = images[idx].src;
}