var items = null;
var idx = 0;
var down = true;
var time = 3000;

$(document).ready(function() {
    items = $('#main img');
    idx = items.length - 1;
    down = false;

    if (items.length > 0) {
        animate();
    }

});

function animate() {
    if (idx >= items.length) {
        down = false;
        idx = items.length - 1;
    }
    else if (idx <= 0) {
        down = true;
        idx = 1;
    }
    if (down) {
        $(items[idx++]).animate({ opacity: 1 }, time, function() { animate() });
    }
    else {
        $(items[idx--]).animate({ opacity: 0 }, time, function() { animate() });
    }
}