var obrazky = new Array( 'img/head-pic-1.jpg', 'img/head-pic-2.jpg' );
    var obrazku = 2;
    var t=setTimeout("zmenObrazek(1)",10000);
    
    function zmenObrazek( img_no ){
          document.getElementById('head-pic').src=obrazky[ img_no ];
          img_no++;
          img_no = img_no % obrazku;
          t=setTimeout("zmenObrazek("+img_no+")",10000);
    }